CC Animations: Port Impl-only-scrolling to use compositor animation timelines.

cc/test/animation_timelines_test_common.cc

Index: cc/test/animation_timelines_test_common.cc
diff --git a/cc/test/animation_timelines_test_common.cc b/cc/test/animation_timelines_test_common.cc
index ea9d0b00cd65295a9f537d8370af5b1b8e49bd0a..9066dce1ce0613ad44464c75ceca068e5d69787b 100644
--- a/cc/test/animation_timelines_test_common.cc
+++ b/cc/test/animation_timelines_test_common.cc
@@ -98,6 +98,11 @@ void TestHostClient::SetLayerScrollOffsetMutated(
   layer->set_scroll_offset(scroll_offset);
 }
 
+gfx::ScrollOffset TestHostClient::GetScrollOffsetForAnimation(
+    int layer_id) const {
+  return gfx::ScrollOffset();
+}
+
 void TestHostClient::RegisterLayer(int layer_id, LayerTreeType tree_type) {
   LayerIdToTestLayer& layers_in_tree = tree_type == LayerTreeType::ACTIVE
                                            ? layers_in_active_tree_