[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

Committed: https://crrev.com/6f454aa1a8891125583d5b16e1b45656643df182
Cr-Commit-Position: refs/heads/master@{#30637}

[Michael Lippautz, 2015-09-08 14:09:53]

lgtm

[Michael Starzinger, 2015-09-08 14:17:35]

The CQ bit was checked by mstarzinger@chromium.org

[commit-bot: I haz the power, 2015-09-08 14:17:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1319153006/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1319153006/1

[commit-bot: I haz the power, 2015-09-08 14:42:34]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-08 14:43:00]

Patchset 1 (id:??) landed as
https://crrev.com/6f454aa1a8891125583d5b16e1b45656643df182
Cr-Commit-Position: refs/heads/master@{#30637}