gpu: Allow fences to check whether a flush has occurred

ui/gl/gl_fence.h

Index: ui/gl/gl_fence.h
diff --git a/ui/gl/gl_fence.h b/ui/gl/gl_fence.h
index c1967ec9c43df4666a17c56b6a6f7f84c45b5a98..2abe1530e5d2df85193cf7dc29069df054195b1c 100644
--- a/ui/gl/gl_fence.h
+++ b/ui/gl/gl_fence.h
@@ -16,11 +16,15 @@ class GL_EXPORT 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;