We can replace this method with a base class method which uses our
implementation of "map". This allows us to reuse setData in other
mappable buffer storage types.

BUG=angle:912

Change-Id: I7848cb9440b806364d9b2a6e2251323e53206921
Reviewed-on: https://chromium-review.googlesource.com/252962Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>