WebGL: Don't destroy mailbox textures in the destructor until they're released.

public/platform/WebExternalTextureMailbox.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 26 matching lines @@
     signed char name[64];
     unsigned syncPoint;
 
     WebExternalTextureMailbox()
         : syncPoint(0)
     {
         memset(name, 0, sizeof(name));
     }
 };
 
+inline bool nameEquals(const WebExternalTextureMailbox& a, const WebExternalTextureMailbox& b)
+{
+    return !memcmp(a.name, b.name, sizeof(a.name));
+}
+
 } // namespace blink
 
 #endif // WebExternalTextureMailbox_h