This moves the client attribs check into a shared place. This cleans up
some of the surrounding code. Also get rid of the vertex count
parameter.

Bug: angleproject:1391
Change-Id: I9c688895c2cc5650d7b395497d69033093916f21
Reviewed-on: https://chromium-review.googlesource.com/1150517
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>