Make tile size a function of the device scale factor.

cc/trees/layer_tree_host_impl.h

Index: cc/trees/layer_tree_host_impl.h
diff --git a/cc/trees/layer_tree_host_impl.h b/cc/trees/layer_tree_host_impl.h
index fad9db7ba8d33337901522ded7c7a40b7f1755e6..facba12d3d185409fd158dcfa49a7c8cbcb9e986 100644
--- a/cc/trees/layer_tree_host_impl.h
+++ b/cc/trees/layer_tree_host_impl.h
@@ -329,8 +329,12 @@ class CC_EXPORT LayerTreeHostImpl
   // 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;