cc: Simplify the logic for deciding to update tile priorities.

cc/resources/picture_layer_tiling.h

Index: cc/resources/picture_layer_tiling.h
diff --git a/cc/resources/picture_layer_tiling.h b/cc/resources/picture_layer_tiling.h
index 622714779739c9e0188c7779ada3ecd8400a0e56..80863512e12173bf4abed9bc536b7a7e66fe3915 100644
--- a/cc/resources/picture_layer_tiling.h
+++ b/cc/resources/picture_layer_tiling.h
@@ -136,8 +136,7 @@ class CC_EXPORT PictureLayerTiling {
       float current_layer_contents_scale,
       const gfx::Transform& last_screen_transform,
       const gfx::Transform& current_screen_transform,
-      int current_source_frame_number,
-      double current_frame_time,
+      double current_frame_time_in_seconds,
       bool store_screen_space_quads_on_tiles,
       size_t max_tiles_for_interest_area);
 
@@ -146,6 +145,10 @@ class CC_EXPORT PictureLayerTiling {
   // also updates the pile on each tile to be the current client's pile.
   void DidBecomeActive();
 
+  bool NeedsUpdateForFrameAtTime(double frame_time_in_seconds) {
+    return frame_time_in_seconds != last_impl_frame_time_in_seconds_;
+  }
+
   scoped_ptr<base::Value> AsValue() const;
 
   static gfx::Rect ExpandRectEquallyToAreaBoundedBy(
@@ -176,8 +179,7 @@ class CC_EXPORT PictureLayerTiling {
   gfx::Rect live_tiles_rect_;
 
   // State saved for computing velocities based upon finite differences.
-  int last_source_frame_number_;
-  double last_impl_frame_time_;
+  double last_impl_frame_time_in_seconds_;
 
   friend class CoverageIterator;