StateManagerGL: Use dirty bits for VAO bindings. Delay binding the VAO in VertexArrayGL::syncDrawState until we actually need to make glVertexAttrib calls. BUG=angleproject:2188 Change-Id: Ib7d22d641c9f19c639ba8c596bff6bc7de952e7f Reviewed-on: https://chromium-review.googlesource.com/919503 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Luc Ferron <lucferron@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>