This missing clear was costing quite a bit of performance on any
use case that called for a lot of state changes.

BUG=angleproject:2188

Change-Id: I98fa6c21728c1655fa9e1019ddc10ccfbdb594d8
Reviewed-on: https://chromium-review.googlesource.com/722422Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>