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

public/platform/WebExternalTextureMailbox.h

Index: public/platform/WebExternalTextureMailbox.h
diff --git a/public/platform/WebExternalTextureMailbox.h b/public/platform/WebExternalTextureMailbox.h
index d8703c6a949158ba8711d5d6277168bea11fcae3..423e4179b0320b4cab87dcc75289cc102774a96c 100644
--- a/public/platform/WebExternalTextureMailbox.h
+++ b/public/platform/WebExternalTextureMailbox.h
@@ -44,6 +44,11 @@ struct WebExternalTextureMailbox {
     }
 };
 
+inline bool nameEquals(const WebExternalTextureMailbox& a, const WebExternalTextureMailbox& b)
+{
+    return !memcmp(a.name, b.name, sizeof(a.name));
+}
+
 } // namespace blink
 
 #endif // WebExternalTextureMailbox_h