cc: Introduce CompositorMode enum.

cc/trees/compositor_mode.h

+// Copyright 2015 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_COMPOSITOR_MODE_H_
+#define CC_TREES_COMPOSITOR_MODE_H_
+
+namespace cc {
+
+// The LayerTreeHost uses the CompositorMode to determine the current mode of
+// operation, which is needed to:
+// 1) Safely cast Proxy to SingleThreadProxy to allow operations only supported
+// in SingleThreaded mode.
+// 2) Make decisions restricted to either browser(SingleThreaded) or renderer
+// compositors(Threaded).
+enum CompositorMode {
+  SingleThreaded,
+  Threaded,
+};
+
+}  // namespace cc
+
+#endif  // CC_TREES_COMPOSITOR_MODE_H_