The Vulkan and GL back-ends both had a client attributes mask.
This consolidates them into the front-end, where it can also
be used in the validation layer.

Also includes a fix which was incorrectly setting the enabled
mask in setVertexAttribFormatImpl.

Bug: angleproject:1391
Change-Id: I5e45c1e2a56b30a36dec1482d170592c30a16d40
Reviewed-on: https://chromium-review.googlesource.com/1008272
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>