feat(material/theming): Add APIs to get density info from theme

Author: mmalerba

src/material/_index.scss

@@ -145,4 +145,5 @@ list-density, list-base;
 
 // New theming APIs, currently in development:
 @forward './core/theming/inspection' as private-* show private-get-theme-version,
-  private-get-theme-type, private-get-theme-color, private-get-theme-typography;
+  private-get-theme-type, private-get-theme-color, private-get-theme-typography,
+  private-get-theme-density;

src/material/core/theming/_inspection.scss

@@ -149,6 +149,18 @@ $_typography-properties: (font, font-family, line-height, font-size, letter-spac
   @return map.get($theme, $_internals, typography-tokens, (mdc, typography), $token-name);
 }
 
+/// Gets the density scale from a theme object.
+/// @param {Map} $theme The theme
+/// @return {Number} The density scale.
+@function get-theme-density($theme) {
+  $err: _validate-theme-object($theme);
+  @if $err {
+    // TODO(mmalerba): implement for old style theme objects.
+    @error #{'get-theme-density does not support legacy theme objects.'};
+  }
+  @return map.get($theme, $_internals, density-scale);
+}
+
 /// Gets the set of tokens from the given theme, limited to those affected by the requested theming
 /// systems.
 /// @param {Map} $theme The theme to get tokens from.

src/material/core/theming/tests/theming-inspection-api.spec.ts

@@ -259,9 +259,20 @@ describe('theming inspection api', () => {
       transpile(`
         $theme: ${defineM3Theme()};
         div {
-          font: mat.private-get-theme-typography($theme, body-small, text-transform);
+          text-transform: mat.private-get-theme-typography($theme, body-small, text-transform);
         }
       `),
     ).toThrowError(/Valid typography properties are:.*Got: text-transform/);
   });
+
+  it('should get density scale', () => {
+    expect(
+      transpile(`
+        $theme: ${defineM3Theme()};
+        div {
+          --density-scale: #{mat.private-get-theme-density($theme)};
+        }
+      `),
+    ).toMatch('--density-scale: 0;');
+  });
 });