cc: Introduce embedded_surfaces in metadata for surface Ids in draw quads

cc/output/compositor_frame_metadata.h

Index: cc/output/compositor_frame_metadata.h
diff --git a/cc/output/compositor_frame_metadata.h b/cc/output/compositor_frame_metadata.h
index 98bed560715476f4dc71382cdc383574ba08b86b..a3ebfc26483ea2f7a2382114e1db2dfc243622c9 100644
--- a/cc/output/compositor_frame_metadata.h
+++ b/cc/output/compositor_frame_metadata.h
@@ -77,8 +77,17 @@ class CC_EXPORT CompositorFrameMetadata {
   std::vector<ui::LatencyInfo> latency_info;
 
   // This is the set of Surfaces that are referenced by this frame.
+  // Note: this includes occluded and clipped surfaces and surfaces that may
+  // be accessed by this CompositorFrame in the future.
+  // TODO(fsamuel): In the future, a generalized frame eviction system will
+  // determine which surfaces to retain and which to evict. It will likely
+  // be unnecessary for the embedder to explicitly specify which surfaces to
+  // retain. Thus, this field will likely go away.
   std::vector<SurfaceId> referenced_surfaces;
 
+  // This is the set of SurfaceIds embedded in DrawQuads.
+  std::vector<SurfaceId> embedded_surfaces;
+
   // This indicates whether this CompositorFrame can be activated before
   // dependencies have been resolved.
   bool can_activate_before_dependencies = true;