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

cc/damage_tracker.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/damage_tracker.h"
 
 #include "cc/layer_impl.h"
 #include "cc/layer_tree_host_common.h"
 #include "cc/math_util.h"
 #include "cc/render_surface_impl.h"
@@ skipping 144 matching lines @@
         return gfx::RectF();
 
     gfx::RectF ret = iter->second;
     m_currentRectHistory->erase(iter);
     return ret;
 }
 
 void DamageTracker::saveRectForNextFrame(int layerID, const gfx::RectF& targetSpaceRect)
 {
     // This layer should not yet exist in next frame's history.
-    DCHECK(layerID > 0);
+    DCHECK(layerID > 0 || layerID == -1 || layerID == -2);

[Ian Vollick, 2013/01/15 20:33:48]

I'll make a more detailed proposal about how to handle layer id's and impl-only
layers later, but if we were to go with my approach, I think DCHECK(layerID)
would be more appropriate here.

     DCHECK(m_nextRectHistory->find(layerID) == m_nextRectHistory->end());
     (*m_nextRectHistory)[layerID] = targetSpaceRect;
 }
 
 gfx::RectF DamageTracker::trackDamageFromActiveLayers(const std::vector<LayerImpl*>& layerList, int targetSurfaceLayerID)
 {
     gfx::RectF damageRect = gfx::RectF();
 
     for (unsigned layerIndex = 0; layerIndex < layerList.size(); ++layerIndex) {
         // Visit layers in back-to-front order.
@@ skipping 156 matching lines @@
 
     // If the layer has a background filter, this may cause pixels in our surface to be expanded, so we will need to expand any damage
     // at or below this layer. We expand the damage from this layer too, as we need to readback those pixels from the surface with only
     // the contents of layers below this one in them. This means we need to redraw any pixels in the surface being used for the blur in
     // this layer this frame.
     if (layer->backgroundFilters().hasFilterThatMovesPixels())
         expandDamageRectInsideRectWithFilters(targetDamageRect, surfaceRectInTargetSpace, layer->backgroundFilters());
 }
 
 }  // namespace cc