gpu: Allow fences to check whether a flush has occurred

ui/gl/gl_fence.h

 // Copyright (c) 2012 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 UI_GL_GL_FENCE_H_
 #define UI_GL_GL_FENCE_H_
 
 #include "base/basictypes.h"
 #include "ui/gl/gl_export.h"
 
 namespace gfx {
 
 class GL_EXPORT GLFence {
  public:
   GLFence();
   virtual ~GLFence();
 
   static GLFence* Create();
+
   // Creates a fence that is not guaranteed to signal until the current context
-  // is flushed. Use with caution.
+  // is flushed. It is illegal to call Client/ServerWait() on a fence without
+  // having explicitly called glFlush() or glFinish() in the originating context.
   static GLFence* CreateWithoutFlush();
+
   virtual bool HasCompleted() = 0;
   virtual void ClientWait() = 0;
+
   // Will block the server if supported, but might fall back to blocking the
   // client.
   virtual void ServerWait() = 0;
 
  protected:
   static bool IsContextLost();
 
  private:
   DISALLOW_COPY_AND_ASSIGN(GLFence);
 };
 
 }  // namespace gfx
 
 #endif  // UI_GL_GL_FENCE_H_