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/217330Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>