This also changes Heap::AllocateExternalBackingStore to avoid GC
inside AlwaysAllocateScope.

Bug: chromium:1042566
Change-Id: Ifa8fe8227af2d6492dbb4f9c52f43754d44926fb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2060295
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66298}