This removes the DisallowAllocationFailure assertion scope which mostly
coincided with the AlwaysAllocateScope anyways. Access to the bitfield
in the Isolate was not synchronized and hence the AlwaysAllocateScope
was not thread-safe in debug mode, now it is.

R=mlippautz@chromium.org

Review URL: https://codereview.chromium.org/1319153006

Cr-Commit-Position: refs/heads/master@{#30637}