cc: Don't subtract transparent overlay from damage.

cc/output/overlay_candidate.h

Index: cc/output/overlay_candidate.h
diff --git a/cc/output/overlay_candidate.h b/cc/output/overlay_candidate.h
index 9c346a848319d4c09f6bb4fdfb66da2fe083761b..8c5d6e26b7962c8fe8a98e555562b8f02ffe8ec0 100644
--- a/cc/output/overlay_candidate.h
+++ b/cc/output/overlay_candidate.h
@@ -67,6 +67,8 @@ class CC_EXPORT OverlayCandidate {
   gfx::Rect clip_rect;
   // If the quad is clipped after composition.
   bool is_clipped;
+  // If the quad doesn't require blending.
+  bool is_opaque;
   // True if the texture for this overlay should be the same one used by the
   // output surface's main overlay.
   bool use_output_surface_for_resource;