This makes it a bit simpler to work with the prepareIndexData method.
Also removes the mRendererClass member of the index data manager
since it was no longer necessary.

BUG=angleproject:2229

Change-Id: I03768b104da21f57c499239bbbb5f716efb0fd48
Reviewed-on: https://chromium-review.googlesource.com/765261
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>