Add an Invalidate method to GLFence.

ui/gl/gl_fence_apple.cc

 // Copyright 2014 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.
 
 #include "ui/gl/gl_fence_apple.h"
 
 #include "ui/gl/gl_bindings.h"
 
 namespace gl {
 
@@ skipping 13 matching lines @@
   DCHECK(glIsFenceAPPLE(fence_));
   glFinishFenceAPPLE(fence_);
 }
 
 void GLFenceAPPLE::ServerWait() {
   DCHECK(glIsFenceAPPLE(fence_));
   ClientWait();
 }
 
 GLFenceAPPLE::~GLFenceAPPLE() {
-  DCHECK(glIsFenceAPPLE(fence_));
-  glDeleteFencesAPPLE(1, &fence_);
+  if (fence_) {
+    DCHECK(glIsFenceAPPLE(fence_));
+    glDeleteFencesAPPLE(1, &fence_);
+  }
+}
+
+void GLFenceAPPLE::Invalidate() {
+  fence_ = 0;
 }
 
 }  // namespace gl