cc: Remove SurfaceDependencies type

cc/surfaces/pending_frame_observer.h

 // 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;
+      const base::flat_set<SurfaceId>& added_dependencies,
+      const base::flat_set<SurfaceId>& 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_