D3D: Add helper for index translation type. 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>