This math can live in a single place in the VertexDataManager. This
cleans up the code and paves the way for future optimizations.

BUG=angleproject:959

Change-Id: I7138c6e080d9c3d6507b55d981bfb62c2590a2a8
Reviewed-on: https://chromium-review.googlesource.com/277282Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>