Rename VertexArrayImpl::mData to mState. Refactoring change only. BUG=angleproject:1898 Change-Id: I9f55651f923ff930c395a9bb575b4f86ad5d9cbd Reviewed-on: https://chromium-review.googlesource.com/707689 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>