EmbedderTest: templatise GetEmbedderContext (flutter#56709)

In many embedder tests, we want to get at the appropriate backend-specific `EmbedderTestContext` subclass (`EmbedderTestContextGL`, etc.) in order to make backend-specific setup calls such as `SetGLFBOCallback()` or others. Formerly, this required casting the returned `EmbedderTestContext&` to the appropriate subclass in each test.

This templatises the `GetEmbedderContext()` method to return the appropriate backend-specific subclass directly.

Issue: flutter#158998

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: cbracken

shell/platform/embedder/tests/embedder_a11y_unittests.cc

@@ -33,8 +33,8 @@ constexpr static char kTooltip[] = "tooltip";
 
 TEST_F(EmbedderTest, CannotProvideMultipleSemanticsCallbacks) {
   {
-    EmbedderConfigBuilder builder(
-        GetEmbedderContext(EmbedderTestContextType::kSoftwareContext));
+    auto& context = GetEmbedderContext<EmbedderTestContextSoftware>();
+    EmbedderConfigBuilder builder(context);
     builder.SetSurface(SkISize::Make(1, 1));
     builder.GetProjectArgs().update_semantics_callback =
         [](const FlutterSemanticsUpdate* update, void* user_data) {};
@@ -46,8 +46,8 @@ TEST_F(EmbedderTest, CannotProvideMultipleSemanticsCallbacks) {
   }
 
   {
-    EmbedderConfigBuilder builder(
-        GetEmbedderContext(EmbedderTestContextType::kSoftwareContext));
+    auto& context = GetEmbedderContext<EmbedderTestContextSoftware>();
+    EmbedderConfigBuilder builder(context);
     builder.SetSurface(SkISize::Make(1, 1));
     builder.GetProjectArgs().update_semantics_callback2 =
         [](const FlutterSemanticsUpdate2* update, void* user_data) {};
@@ -61,8 +61,8 @@ TEST_F(EmbedderTest, CannotProvideMultipleSemanticsCallbacks) {
   }
 
   {
-    EmbedderConfigBuilder builder(
-        GetEmbedderContext(EmbedderTestContextType::kSoftwareContext));
+    auto& context = GetEmbedderContext<EmbedderTestContextSoftware>();
+    EmbedderConfigBuilder builder(context);
     builder.SetSurface(SkISize::Make(1, 1));
     builder.GetProjectArgs().update_semantics_callback =
         [](const FlutterSemanticsUpdate* update, void* user_data) {};
@@ -76,8 +76,8 @@ TEST_F(EmbedderTest, CannotProvideMultipleSemanticsCallbacks) {
   }
 
   {
-    EmbedderConfigBuilder builder(
-        GetEmbedderContext(EmbedderTestContextType::kSoftwareContext));
+    auto& context = GetEmbedderContext<EmbedderTestContextSoftware>();
+    EmbedderConfigBuilder builder(context);
     builder.SetSurface(SkISize::Make(1, 1));
     builder.GetProjectArgs().update_semantics_callback2 =
         [](const FlutterSemanticsUpdate2* update, void* user_data) {};
@@ -98,7 +98,7 @@ TEST_F(EmbedderA11yTest, A11yTreeIsConsistentUsingV3Callbacks) {
   GTEST_SKIP() << "This test crashes on Fuchsia. https://fxbug.dev/87493 ";
 #else
 
-  auto& context = GetEmbedderContext(EmbedderTestContextType::kSoftwareContext);
+  auto& context = GetEmbedderContext<EmbedderTestContextSoftware>();
 
   fml::AutoResetWaitableEvent signal_native_latch;
 
@@ -277,7 +277,7 @@ TEST_F(EmbedderA11yTest, A11yStringAttributes) {
   GTEST_SKIP() << "This test crashes on Fuchsia. https://fxbug.dev/87493 ";
 #else
 
-  auto& context = GetEmbedderContext(EmbedderTestContextType::kSoftwareContext);
+  auto& context = GetEmbedderContext<EmbedderTestContextSoftware>();
 
   fml::AutoResetWaitableEvent signal_native_latch;
 
@@ -397,7 +397,7 @@ TEST_F(EmbedderA11yTest, A11yTreeIsConsistentUsingV2Callbacks) {
   GTEST_SKIP() << "This test crashes on Fuchsia. https://fxbug.dev/87493 ";
 #else
 
-  auto& context = GetEmbedderContext(EmbedderTestContextType::kSoftwareContext);
+  auto& context = GetEmbedderContext<EmbedderTestContextSoftware>();
 
   fml::AutoResetWaitableEvent signal_native_latch;
 
@@ -570,7 +570,7 @@ TEST_F(EmbedderA11yTest, A11yTreeIsConsistentUsingV2Callbacks) {
 }
 
 TEST_F(EmbedderA11yTest, A11yTreeIsConsistentUsingV1Callbacks) {
-  auto& context = GetEmbedderContext(EmbedderTestContextType::kSoftwareContext);
+  auto& context = GetEmbedderContext<EmbedderTestContextSoftware>();
 
   fml::AutoResetWaitableEvent signal_native_latch;