Introduce separate client and init path for single-threaded cc

cc/trees/layer_tree_host_single_thread_client.h

+// Copyright 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CC_TREES_LAYER_TREE_HOST_SINGLE_THREAD_CLIENT_H_
+#define CC_TREES_LAYER_TREE_HOST_SINGLE_THREAD_CLIENT_H_
+
+namespace cc {
+
+class LayerTreeHostSingleThreadClient {
+ public:
+  // Request that the client schedule a composite.
+  virtual void ScheduleComposite() = 0;
+
+  // Called whenever the compositor posts a SwapBuffers (either full or
+  // partial). After DidPostSwapBuffers(), exactly one of
+  // LayerTreeHostClient::DidCompleteSwapBuffers() or DidAbortSwapBuffers() will
+  // be called, thus these functions can be used to keep track of pending swap
+  // buffers calls for rate limiting.
+  virtual void DidPostSwapBuffers() = 0;
+  virtual void DidAbortSwapBuffers() = 0;
+
+ protected:
+  virtual ~LayerTreeHostSingleThreadClient() {}
+};
+
+}  // namespace cc
+
+#endif  // CC_TREES_LAYER_TREE_HOST_SINGLE_THREAD_CLIENT_H_