Speculatively turn off the `PartitionAllocZeroFill` optimization on macOS.

It might? be the cause of some test flakage.

Bug: 892550
TBR: ajwong
Change-Id: I4ac98e2e5e3d02526ed2c187157d097b82adc756
Reviewed-on: https://chromium-review.googlesource.com/c/1266199
Commit-Queue: Chris Palmer <palmer@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597390}

base/allocator/partition_allocator/partition_bucket.cc

@@ -478,7 +478,10 @@ void* PartitionBucket::SlowPathAlloc(PartitionRootBase* root,
       PartitionExcessiveAllocationSize();
     }
     new_page = PartitionDirectMap(root, flags, size);
+#if !defined(OS_MACOSX)
+    // Turn off the optimization to see if it helps https://crbug.com/892550.
     *is_already_zeroed = true;
+#endif
   } else if (LIKELY(this->SetNewActivePage())) {
     // First, did we find an active page in the active pages list?
     new_page = this->active_pages_head;