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>
