Force use of and cache render surface.

cc/trees/draw_property_utils.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/draw_property_utils.h"
 
 #include <stddef.h>
 
 #include <vector>
 
@@ skipping 540 matching lines @@
     is_clipped = true;
   } else if (render_surface->has_contributing_layer_that_escapes_clip()) {
     // We cannot clip a surface that has a contribuitng layer which escapes the
     // clip.
     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
     // 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.
     const ClipNode* clip_node = clip_tree.Node(render_surface->ClipTreeIndex());
     is_clipped = clip_node->clip_type != ClipNode::ClipType::EXPANDS_CLIP;
   }
   render_surface->SetIsClipped(is_clipped);
 }
 
 static void SetSurfaceDrawOpacity(const EffectTree& tree,
@@ skipping 519 matching lines @@
 void UpdateElasticOverscroll(PropertyTrees* property_trees,
                              const Layer* overscroll_elasticity_layer,
                              const gfx::Vector2dF& elastic_overscroll) {
   UpdateElasticOverscrollInternal(property_trees, overscroll_elasticity_layer,
                                   elastic_overscroll);
 }
 
 }  // namespace draw_property_utils
 
 }  // namespace cc