Add an Invalidate method to GLFence.

ui/gl/gl_fence_arb.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_arb.h"
 
 #include "base/strings/stringprintf.h"
 #include "ui/gl/gl_bindings.h"
 
 namespace gl {
@@ skipping 43 matching lines @@
     LOG(FATAL) << "Failed to wait for GLFence. error code:" << GetGLErrors();
   }
 }
 
 void GLFenceARB::ServerWait() {
   DCHECK_EQ(GL_TRUE, glIsSync(sync_));
   glWaitSync(sync_, 0, GL_TIMEOUT_IGNORED);
 }
 
 GLFenceARB::~GLFenceARB() {
-  DCHECK_EQ(GL_TRUE, glIsSync(sync_));
-  glDeleteSync(sync_);
+  if (sync_) {
+    DCHECK_EQ(GL_TRUE, glIsSync(sync_));
+    glDeleteSync(sync_);
+  }
+}
+
+void GLFenceARB::Invalidate() {
+  sync_ = 0;
 }
 
 }  // namespace gl