Now that there's storage change notifications in the GL front-end we
no longer need to give the back-end access to the angle::Subject. The
Texture object is a special case where it has mirrored dirty bits. To
keep the gl::Texture class notified of when the Impl has dirty bits we
make the TextureImpl class an angle::Subject that is observed by the
gl::Texture class.

This will enable further dirty bits improvements.

Bug: angleproject:2966
Change-Id: Id22da0926f51ff4679e58af3e62903f4d7948915
Reviewed-on: https://chromium-review.googlesource.com/c/1347670Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>