There were two code paths using nearly identical code. Fix this by
generalizing the helper method slightly.

Refactoring change only.

Bug: angleproject:2828
Change-Id: I39caccfaad4880e566e995483b8e8fb46d67d3da
Reviewed-on: https://chromium-review.googlesource.com/1249563Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>