implement the logic to set tile priorities based on current matrix

cc/picture_layer_impl.cc

Index: cc/picture_layer_impl.cc
diff --git a/cc/picture_layer_impl.cc b/cc/picture_layer_impl.cc
index 18095964f70095b5ee16343138d2fc01f5003e4e..7f8f6fdecd40ae39690f9ff77ff6bc0ddc650c0e 100644
--- a/cc/picture_layer_impl.cc
+++ b/cc/picture_layer_impl.cc
@@ -4,6 +4,7 @@
 
 #include "cc/picture_layer_impl.h"
 
+#include "base/time.h"
 #include "cc/layer_tree_host_impl.h"
 #include "cc/math_util.h"
 #include "cc/quad_sink.h"
@@ -13,7 +14,8 @@ namespace cc {
 
 PictureLayerImpl::PictureLayerImpl(int id) :
     LayerImpl(id),
-    tilings_(this) {
+    tilings_(this),
+    last_update_time_(0) {
 }
 
 PictureLayerImpl::~PictureLayerImpl() {
@@ -88,6 +90,18 @@ void PictureLayerImpl::didUpdateTransforms() {
     tilings_.AddTiling(contentsScaleX(), tile_size);
     // TODO(enne): handle invalidations, create new tiles
   }
+
+  gfx::Transform  current_transform = screenSpaceTransform();
+  double current_time =
+      (base::TimeTicks::Now() - base::TimeTicks()).InSecondsF();
+  if (last_update_time_ != 0) {
+    tilings_.UpdateTilePriorities(layerTreeHostImpl()->deviceViewportSize(),
+                                  last_transform_,
+                                  current_transform,
+                                  current_time - last_update_time_);
+  }
+  last_transform_ = current_transform;
+  last_update_time_ = current_time;
 }
 
 scoped_refptr<Tile> PictureLayerImpl::CreateTile(PictureLayerTiling*,