Move client attribs mask to front-end. 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>