Make tile size a function of the device scale factor.

cc/trees/layer_tree_host_impl.h

 // 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.
 
 #ifndef CC_TREES_LAYER_TREE_HOST_IMPL_H_
 #define CC_TREES_LAYER_TREE_HOST_IMPL_H_
 
 #include <stddef.h>
 
 #include <bitset>
@@ skipping 311 matching lines @@
   void UnregisterScrollbarAnimationController(int scroll_layer_id);
   ScrollbarAnimationController* ScrollbarAnimationControllerForId(
       int scroll_layer_id) const;
 
   DrawMode GetDrawMode() const;
 
   // Viewport size in draw space: this size is in physical pixels and is used
   // for draw properties, tilings, quads and render passes.
   gfx::Size DrawViewportSize() const;
 
+  // A screen size for the physical device that is independent from the size of
+  // the compositor (aka the viewport). It is specific to a single tree.
+  gfx::Size GetStableScreenSize(const LayerTreeImpl* tree) const;
+
   // Viewport rect in view space used for tiling prioritization.
-  const gfx::Rect ViewportRectForTilePriority() const;
+  gfx::Rect ViewportRectForTilePriority() const;
 
   // RendererClient implementation.
   void SetFullRootLayerDamage() override;
 
   // TileManagerClient implementation.
   void NotifyReadyToActivate() override;
   void NotifyReadyToDraw() override;
   void NotifyAllTileTasksCompleted() override;
   void NotifyTileStateChanged(const Tile* tile) override;
   std::unique_ptr<RasterTilePriorityQueue> BuildRasterQueue(
@@ skipping 506 matching lines @@
 
   std::unique_ptr<PendingTreeDurationHistogramTimer>
       pending_tree_duration_timer_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerTreeHostImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_HOST_IMPL_H_