Implement pinch-zoom scaling for main-frame scrollbars and pinch-zoom overlay scrollbars.

cc/layer_tree_host_common.cc

 // Copyright 2011 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/layer_tree_host_common.h"
 
 #include <algorithm>
 
 #include "base/debug/trace_event.h"
 #include "cc/heads_up_display_layer_impl.h"
@@ skipping 214 matching lines @@
     // via a render surface or explicitly if the parent preserves 3D), so the
     // entire subtree can be skipped if this layer is fully transparent.
     return !layer->opacity();
 }
 
 static inline bool subtreeShouldBeSkipped(Layer* layer)
 {
     // If the opacity is being animated then the opacity on the main thread is unreliable
     // (since the impl thread may be using a different opacity), so it should not be trusted.
     // In particular, it should not cause the subtree to be skipped.
-    return !layer->opacity() && !layer->OpacityIsAnimating();
+    // Similarly, for layers that might animate opacity using an impl-only
+    // animation, their subtree should also not be skipped.
+    return !layer->opacity() && !layer->OpacityIsAnimating() &&
+           !layer->OpacityCanAnimateOnImplThread();
 }
 
 // Called on each layer that could be drawn after all information from
 // calcDrawProperties has been updated on that layer.  May have some false
 // positives (e.g. layers get this called on them but don't actually get drawn).
 static inline void updateTilePrioritiesForLayer(LayerImpl* layer)
 {
     layer->UpdateTilePriorities();
 
     // Mask layers don't get this call, so explicitly update them so they can
@@ skipping 968 matching lines @@
 
   // At this point, we think the point does hit the touch event handler region on the layer, but we need to walk up
   // the parents to ensure that the layer was not clipped in such a way that the
   // hit point actually should not hit the layer.
   if (pointIsClippedBySurfaceOrClipRect(screenSpacePoint, layerImpl))
      return false;
 
   return true;
 }
 }  // namespace cc