Force use of and cache render surface.

cc/trees/draw_property_utils.cc

Index: cc/trees/draw_property_utils.cc
diff --git a/cc/trees/draw_property_utils.cc b/cc/trees/draw_property_utils.cc
index 893f6baf83a4aaa51f8eaabeb82863cc687bc666..d3d27e711c404c6ca54aec26c61c25511501aeac 100644
--- a/cc/trees/draw_property_utils.cc
+++ b/cc/trees/draw_property_utils.cc
@@ -558,6 +558,8 @@ static void SetSurfaceIsClipped(const ClipTree& clip_tree,
     // There is no clip between between the render surface and its target, so
     // the surface need not be clipped.
     is_clipped = false;
+  } else if (render_surface->ForceRenderSurface()) {
+    is_clipped = false;

[reveman, 2017/05/10 13:06:17]

Please add a comment here to explain why we do this.

[wutao, 2017/05/11 19:36:34]

Done.

   } else {
     // If the clips between the render surface and its target only expand the
     // clips and do not apply any new clip, we need not clip the render surface.