cc: Only allow trees created at the current viewport size to draw.

cc/layer_tree_impl.h

Index: cc/layer_tree_impl.h
diff --git a/cc/layer_tree_impl.h b/cc/layer_tree_impl.h
index b8971f842d4005b67ec399385f818c76477f2253..d71f9679d5b19937dff53f7d3889e52dfa938f91 100644
--- a/cc/layer_tree_impl.h
+++ b/cc/layer_tree_impl.h
@@ -71,8 +71,6 @@ class CC_EXPORT LayerTreeImpl {
   // trivial accessors in a followup patch.
   const LayerTreeDebugState& debug_state() const;
   float device_scale_factor() const;
-  const gfx::Size& device_viewport_size() const;
-  const gfx::Size& layout_viewport_size() const;
   std::string layer_tree_as_text() const;
   DebugRectHistory* debug_rect_history() const;
 
@@ -138,6 +136,10 @@ class CC_EXPORT LayerTreeImpl {
   }
   float sent_page_scale_delta() const { return sent_page_scale_delta_; }
 
+  void SetViewportSize(gfx::Size layout_viewport_size, gfx::Size device_viewport_size);
+  const gfx::Size& layout_viewport_size() const { return layout_viewport_size_; }
+  const gfx::Size& device_viewport_size() const { return device_viewport_size_; }
+
   // Updates draw properties and render surface layer list
   void UpdateDrawProperties(UpdateDrawPropertiesReason reason);
   void set_needs_update_draw_properties() {
@@ -198,6 +200,9 @@ protected:
   float min_page_scale_factor_;
   float max_page_scale_factor_;
 
+  gfx::Size device_viewport_size_;
+  gfx::Size layout_viewport_size_;
+
   typedef base::hash_map<int, LayerImpl*> LayerIdMap;
   LayerIdMap layer_id_map_;