Instead of calling {TryAllocateBackingStore} with
{enable_guard_regions==false}, we just directly call
{array_buffer_allocator->Allocate}.
Drive-by optimization: Don't allocate if the size is 0.

R=titzer@chromium.org

Change-Id: Iabf7af7e0f1bc970c03efcd9ee4c23e5307a7095
Reviewed-on: https://chromium-review.googlesource.com/548398Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46218}