Cache active attribute lists in VertexDataManager. This saves us re-creating vector memory every iteration, and allows us to iterate over much smaller lists of attributes. In the future it could allow us to update the cache more efficiently with state change updates. BUG=angleproject:959 Change-Id: Ie8ae7a31726468dc2184165380f1f3e5e0152936 Reviewed-on: https://chromium-review.googlesource.com/277284 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>