Handles the gl::Framebuffer class and its implementation.

Bug: angleproject:2491
Change-Id: I3b9c0609e9277264ccdb370596500562df3b7d15
Reviewed-on: https://chromium-review.googlesource.com/c/1280743Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>