cc: Split Proxy to eliminate unnecessary dependencies on the impl side

cc/test/layer_test_common.h

Index: cc/test/layer_test_common.h
diff --git a/cc/test/layer_test_common.h b/cc/test/layer_test_common.h
index f801461297c1bee647934e8e82e886ad42ea732b..4cafddddffedd02875f873697e25b1ae33e8d3d4 100644
--- a/cc/test/layer_test_common.h
+++ b/cc/test/layer_test_common.h
@@ -133,7 +133,9 @@ class LayerTestCommon {
     LayerImpl* root_layer() const { return root_layer_impl_.get(); }
     FakeLayerTreeHost* host() { return host_.get(); }
     FakeLayerTreeHostImpl* host_impl() const { return host_->host_impl(); }
-    Proxy* proxy() const { return host_->host_impl()->proxy(); }
+    ThreadVerifier* thread_verifier() const {
+      return host_->host_impl()->thread_verifier();
+    }
     const QuadList& quad_list() const { return render_pass_->quad_list; }
 
    private: