Add artificial limits to the total memory allocated by the NULL backend. 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>