Introduce separate client and init path for single-threaded cc

cc/layers/scrollbar_layer_unittest.cc

Index: cc/layers/scrollbar_layer_unittest.cc
diff --git a/cc/layers/scrollbar_layer_unittest.cc b/cc/layers/scrollbar_layer_unittest.cc
index df12b736686ba879a9528f16f3228f3fcf0ce437..53a9fa87b68228ef4d9735123b2f99e8bad5a04c 100644
--- a/cc/layers/scrollbar_layer_unittest.cc
+++ b/cc/layers/scrollbar_layer_unittest.cc
@@ -512,13 +512,13 @@ TEST_F(ScrollbarLayerTestMaxTextureSize, DelegatingRenderer) {
 
 class MockLayerTreeHost : public LayerTreeHost {
  public:
-  MockLayerTreeHost(LayerTreeHostClient* client,
+  MockLayerTreeHost(FakeLayerTreeHostClient* client,
                     const LayerTreeSettings& settings)
       : LayerTreeHost(client, NULL, settings),
         next_id_(1),
         total_ui_resource_created_(0),
         total_ui_resource_deleted_(0) {
-    Initialize(NULL);
+    InitializeSingleThreaded(client);
   }
 
   virtual UIResourceId CreateUIResource(UIResourceClient* content) OVERRIDE {