Fix (or tag) uses of explicit '64' instead of GL_MAILBOX_SIZE_CHROMIUM

gpu/command_buffer/common/mailbox.h

 // Copyright 2013 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 GPU_COMMAND_BUFFER_MAILBOX_H_
 #define GPU_COMMAND_BUFFER_MAILBOX_H_
 
 #include <string.h>
 
 #include "gpu/command_buffer/common/types.h"
 #include "gpu/gpu_export.h"
 
+// From gl2/gl2ext.h.
+#ifndef GL_MAILBOX_SIZE_CHROMIUM
+#define GL_MAILBOX_SIZE_CHROMIUM 64
+#endif
+
 namespace gpu {
 
 struct GPU_EXPORT Mailbox {
   Mailbox();
   bool IsZero() const;
   void SetZero();
   void SetName(const int8* name);
-  int8 name[64];
+  int8 name[GL_MAILBOX_SIZE_CHROMIUM];
   bool operator<(const Mailbox& other) const {
     return memcmp(this, &other, sizeof other) < 0;
   }
   bool operator==(const Mailbox& other) const {
     return memcmp(this, &other, sizeof other) == 0;
   }
   bool operator!=(const Mailbox& other) const {
     return !operator==(other);
   }
 };
 
 }  // namespace gpu
 
 #endif  // GPU_COMMAND_BUFFER_MAILBOX_H_