Fix gpu::gles2::MailboxName on Windows

It is "struct GPU_EXPORT MailboxName" in
chromium/gpu/command_buffer/service/mailbox_manager.h.
MSVC won't accept class.

Change-Id: I979808f357b32e1d99fe70a1138bf292b54c4e76
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>