Unmap does not flush, so we need to do it explicitly.

BUG=angleproject:2436

Change-Id: Id102825b1329d97965f07b97c227214fa7524a0e
Reviewed-on: https://chromium-review.googlesource.com/980927Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>