Handles the gl::Renderbuffer class and its implementation.

Bug: angleproject:2491
Change-Id: I3ef718b492dc39ba640c8f68ed80b536f89ff378
Reviewed-on: https://chromium-review.googlesource.com/c/1283309Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>