Implement buffer map related entry points and errors. BUG=angle:565 Change-Id: I14cee4eec032d4b6b8d9479da4234b81e4ec6bec Reviewed-on: https://chromium-review.googlesource.com/188202 Reviewed-by: Shannon Woods <shannonwoods@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>