cc: Abstract the LayerTreeHost.

cc/trees/layer_tree_host_pixeltest_tiles.cc

Index: cc/trees/layer_tree_host_pixeltest_tiles.cc
diff --git a/cc/trees/layer_tree_host_pixeltest_tiles.cc b/cc/trees/layer_tree_host_pixeltest_tiles.cc
index b310d95b3c947f9b71816240284210ed73fb9cd2..67342e2115120d01d8e0f45447aa13503516660c 100644
--- a/cc/trees/layer_tree_host_pixeltest_tiles.cc
+++ b/cc/trees/layer_tree_host_pixeltest_tiles.cc
@@ -159,7 +159,7 @@ class LayerTreeHostTilesTestPartialInvalidation
   }
 
   void DidCommitAndDrawFrame() override {
-    switch (layer_tree_host()->source_frame_number()) {
+    switch (layer_tree_host()->SourceFrameNumber()) {
       case 1:
         // We have done one frame, so the layer's content has been rastered.
         // Now we change the picture behind it to record something completely