A couple index data manager and Renderer helper methods didn't need
to modify state or use any local member variables, so we can make
them static to be more accessible from other parts of the code.

BUG=angleproject:1156

Change-Id: I522d0dfd396f0fed7b5087e90dcb70a0f9502a75
Reviewed-on: https://chromium-review.googlesource.com/616782Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>