Use a helper method isValidIndex to preserve ASSERT checks.

Refactoring patch only.

BUG=angle:741

Change-Id: Ie19fa21db51cd0239a3b391de362584a9fbab2df
Reviewed-on: https://chromium-review.googlesource.com/222268Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>