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.

The re-land fixes a crash with WebGL related to element array buffers.

BUG=angleproject:1040
TEST=WebGL CTS, end2end_tests, unittests

Change-Id: I9b82e06825bf95f0fc2b7c7427e1eb6dd257c1ee
Reviewed-on: https://chromium-review.googlesource.com/290044Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>