Reusing base::IdType<...> to implement SurfaceId.

cc/test/surface_aggregator_test_helpers.h

Index: cc/test/surface_aggregator_test_helpers.h
diff --git a/cc/test/surface_aggregator_test_helpers.h b/cc/test/surface_aggregator_test_helpers.h
index ae3347c74a8a9affcf2f1fe064646ec6866a8483..394b750d5cb8d161c57a2bfc216d828185845331 100644
--- a/cc/test/surface_aggregator_test_helpers.h
+++ b/cc/test/surface_aggregator_test_helpers.h
@@ -24,27 +24,9 @@ typedef std::vector<scoped_ptr<RenderPass>> RenderPassList;
 namespace test {
 
 struct Quad {
-  static Quad SolidColorQuad(SkColor color) {
-    Quad quad;
-    quad.material = DrawQuad::SOLID_COLOR;
-    quad.color = color;
-    return quad;
-  }
-
-  static Quad SurfaceQuad(SurfaceId surface_id, float opacity) {
-    Quad quad;
-    quad.material = DrawQuad::SURFACE_CONTENT;
-    quad.opacity = opacity;
-    quad.surface_id = surface_id;
-    return quad;
-  }
-
-  static Quad RenderPassQuad(RenderPassId id) {
-    Quad quad;
-    quad.material = DrawQuad::RENDER_PASS;
-    quad.render_pass_id = id;
-    return quad;
-  }
+  static Quad SolidColorQuad(SkColor color);
+  static Quad SurfaceQuad(SurfaceId surface_id, float opacity);
+  static Quad RenderPassQuad(RenderPassId id);
 
   DrawQuad::Material material;
   // Set when material==DrawQuad::SURFACE_CONTENT.
@@ -56,7 +38,7 @@ struct Quad {
   RenderPassId render_pass_id;
 
  private:
-  Quad() : material(DrawQuad::INVALID), opacity(1.f), color(SK_ColorWHITE) {}
+  Quad();
 };
 
 struct Pass {