This method doesn't need to call through to the VertexBuffer class,
since in only really depends on vertex format caps. This makes the
code local to the VertexDataManager, the only place it is used.

Refactoring patch only, for VertexArray11 dirty bits.

BUG=angleproject:1327

Change-Id: I315313a72a00186930d7f9b1091ccb91f37f9f96
Reviewed-on: https://chromium-review.googlesource.com/329740Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>