Implement service-side surface synchronization

cc/surfaces/pending_frame_observer.h

Index: cc/surfaces/pending_frame_observer.h
diff --git a/cc/surfaces/pending_frame_observer.h b/cc/surfaces/pending_frame_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..ec7acab2e2a83e490a956f8ecf0501c1d5919092
--- /dev/null
+++ b/cc/surfaces/pending_frame_observer.h
@@ -0,0 +1,41 @@
+// Copyright 2017 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_SURFACES_PENDING_FRAME_OBSERVER_H_
+#define CC_SURFACES_PENDING_FRAME_OBSERVER_H_
+
+#include "base/containers/flat_set.h"
+#include "cc/surfaces/surface_id.h"
+#include "cc/surfaces/surfaces_export.h"
+
+namespace cc {
+
+using SurfaceDependencies = base::flat_set<SurfaceId>;
+
+class Surface;
+
+// Clients that wish to observe when the state of a pending CompostiorFrame
+// changes should implement this class.
+class CC_SURFACES_EXPORT PendingFrameObserver {
+ public:
+  // Called when a CompositorFrame within |surface| has activated.
+  virtual void OnSurfaceActivated(Surface* surface) = 0;
+
+  // Called when the dependencies of a pending CompositorFrame within |surface|
+  // have changed.
+  virtual void OnSurfaceDependenciesChanged(
+      Surface* surface,
+      const SurfaceDependencies& added_dependencies,
+      const SurfaceDependencies& removed_dependencies) = 0;
+
+  // Called when |surface| is being destroyed.
+  virtual void OnSurfaceDiscarded(Surface* surface) = 0;
+
+ protected:
+  virtual ~PendingFrameObserver() = default;
+};
+
+}  // namespace cc
+
+#endif  // CC_SURFACES_PENDING_FRAME_OBSERVER_H_