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>