The static promotion somehow slipped into the direct usage check,
meaning we would never promote dynamic index data to static, even
if unchanged for many frames. Fix this by putting the update back
in the correct place.

BUG=angleproject:1334

Change-Id: I90897fcd2f6a2b231ccef9f1fa044d276b238b56
Reviewed-on: https://chromium-review.googlesource.com/331388Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>