Refactors gl::Buffer and implementation.

Bug: angleproject:2491
Change-Id: Ic6860dac2faf33067965e7117ea8e98dc6a8dc7a
Reviewed-on: https://chromium-review.googlesource.com/c/1283310Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>