Fuzzer tests were capable of allocating very large chunks of memory by
calling glBufferData with a null pointer.  This sets a limit before the
NULL backend starts returning out of memory GL errors.

BUG=602737

Change-Id: Ic53ebcf999f951b96c1df82e4db57e949d03c908
Reviewed-on: https://chromium-review.googlesource.com/441184
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>