Crash the renderer when XHR fails to allocate an ArrayBuffer on response attribute access

See http://src.chromium.org/viewvc/blink?view=revision&revision=165278.
This changed behavior of ArrayBuffer allocation on OOM from crashing to
returning null. Add ASSERT to keep old behavior.

BUG=357356

Review URL: https://codereview.chromium.org/242713002

git-svn-id: svn://svn.chromium.org/blink/trunk@171930 bbb929c8-8fbe-4397-9dbb-9b2b20218538