VertexArray: Store bound attributes mask in binding. Making the bound attributes mask part of the struct keeps the style a bit more consistent. It will also make updating the cache variable a bit easier. Bug: angleproject:1391 Change-Id: I2b0ba1161d2579d95ddfbae7a05e490c9644a126 Reviewed-on: https://chromium-review.googlesource.com/1150513 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>