Add a helper to replace FBO attachment's getSerial. Ideally we could store a render target serial in the render target itself. For now, this helper function allows us to finally stop exposing rx::RenderTarget from gl::Texture. BUG=angle:732 Change-Id: I4ffc5e0af6f0542212ad4e74adced21ebbd89462 Reviewed-on: https://chromium-review.googlesource.com/217330 Reviewed-by: Shannon Woods <shannonwoods@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>