[heap] Remove obsolete DisallowAllocationFailure scope.

[heap] Remove obsolete DisallowAllocationFailure scope.

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}