to be consistent with how we start vertex attributes. A null pointer indicates we're using the streaming buffer. Will also aid the dirty state bits refactor. BUG=angleproject:1040 TEST=WebGL CTS, end2end_tests, unittests Change-Id: I7a9167282b60dbe0cbb9f0e5d9e3770890ffeb71 Reviewed-on: https://chromium-review.googlesource.com/284619Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>
