Revert "Speculatively turn off the `PartitionAllocZeroFill` optimization on macOS."

This reverts commit a7527ee7.

Reason for revert: It was a speculative workaround that may not be needed.

Original change's description:
> 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}

TBR=ajwong@chromium.org,palmer@chromium.org,mek@chromium.org,haraken@chromium.org,mastiz@chromium.org

Bug: 892550
Change-Id: I55bc9c2869f506cd0f3fa73103c715982b598599
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2031709Reviewed-by: Chris Palmer <palmer@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737206}

base/allocator/partition_allocator/partition_bucket.cc

@@ -481,10 +481,7 @@ void* PartitionBucket::SlowPathAlloc(PartitionRootBase* root,
       PartitionExcessiveAllocationSize(size);
     }
     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(SetNewActivePage())) {
     // First, did we find an active page in the active pages list?
     new_page = active_pages_head;