Running dEQP-GLES3.functional.transform_feedback would trigger a few
assertion failures because of some regressions that crept into our
code.

Among the regressions is assuming we can't map a buffer that has no
underlying storage object. This is valid since we sometimes allow
setData without creating a native storage.

Additionally we should free our existing data store on a new call
to SetData, to prevent old but "new" data revisions from complicating
our storage.

BUG=angle:679

Change-Id: Icd96a8a55a2b957d29382d8b8bbd7e8a7ab0cf65
Reviewed-on: https://chromium-review.googlesource.com/204342Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>