Implement state and interface for map in gl::Buffer. Mostly these calls pass through to BufferStorage. BUG=angle:565 Change-Id: I38ab7d4f49bfad940773428f5bb330de55d3f573 Reviewed-on: https://chromium-review.googlesource.com/187080 Reviewed-by: Nicolas Capens <nicolascapens@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>