Initial surface aggregator implementation

cc/quads/shared_quad_state.h

Index: cc/quads/shared_quad_state.h
diff --git a/cc/quads/shared_quad_state.h b/cc/quads/shared_quad_state.h
index 9098bfb7c366ea6787af33a5144f583804b54706..9c10f6cd581b662142bae5721c853c54874649e3 100644
--- a/cc/quads/shared_quad_state.h
+++ b/cc/quads/shared_quad_state.h
@@ -17,6 +17,11 @@ class Value;
 
 namespace cc {
 
+// SharedQuadState holds a set of properties that are common across multiple
+// DrawQuads. It's purely an optimization - the properties behave in exactly the
+// same way as if they were replicated on each DrawQuad. A given SharedQuadState
+// can only be shared by DrawQuads that are adjacent in their RenderPass'
+// QuadList.
 class CC_EXPORT SharedQuadState {
  public:
   static scoped_ptr<SharedQuadState> Create();