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 43120259e3db8674761c76254eedd46c3418a0a0..a91d9318178e2cef0fc9a6ef1770486ed5c6d1c0 100644
--- a/cc/trees/draw_property_utils.cc
+++ b/cc/trees/draw_property_utils.cc
@@ -562,9 +562,13 @@ static void SetSurfaceIsClipped(const ClipTree& clip_tree,
     is_clipped = false;
   } else if (render_surface->ClipTreeIndex() ==
              render_surface->render_target()->ClipTreeIndex()) {
-    // There is no clip between between the render surface and its target, so
+    // There is no clip between the render surface and its target, so
     // the surface need not be clipped.
     is_clipped = false;
+  } else if (render_surface->ForceRenderSurface()) {

[weiliangc, 2017/06/07 21:19:01]

I don't think this is where "force no clip" should happen.

My understanding for the wanted behavior is that:
1. Everything contributing to the render surface shouldn't be clipped by
anything outside the render surface to be fully visible/fully rastered. This
would be similar to how copy request is handled in the LayerVisibleRect
function. Another part of this work is to make sure when we force render surface
the drawable_content_space ends up sum of all contributing content without being
clipped.

2. The render surface itself, when drawing to the screen, would still be
clipped. So for render surface ForceRenderSurface we shouldn't set is_clipped
false here.

[wutao, 2017/06/09 02:31:33]

Done.

+    // Avoid clipping when force use of render surface. We might need the whole
+    // surface during some animations.
+    is_clipped = false;
   } 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.