After a release() or destroy() it is now possible continue using a
DynamicBuffer because we reset the internal variables and the next
allocate() restarts everything.  Also removed the valid() method as it
wasn't very useful and made reuse more complicated.  No functional change.

BUG=angleproject:2405

Change-Id: I5ca9b758fb300618f6578ebc406494ad281c1622
Reviewed-on: https://chromium-review.googlesource.com/1144229
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>