Revert "[PartitionAlloc] Don't poison memory on free."

This reverts commit 99391f02.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> [PartitionAlloc] Don't poison memory on free.
> 
> It might? be the cause of a perf regression.
> 
> Bug: 882471,680657
> Change-Id: Iec34a348426ccfdf5864a3b2cbe0c377da20ac0e
> Reviewed-on: https://chromium-review.googlesource.com/1220468
> Commit-Queue: Chris Palmer <palmer@chromium.org>
> Reviewed-by: Emil A Eklund <eae@chromium.org>
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#590508}

TBR=palmer@chromium.org,eae@chromium.org,haraken@chromium.org

Change-Id: I6c687b8cb2b8aa0bc3de682c1c39f430b1f6b05c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 882471, 680657
Reviewed-on: https://chromium-review.googlesource.com/1220815Reviewed-by: Chris Palmer <palmer@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590545}

base/allocator/partition_allocator/partition_page.h

@@ -209,9 +209,13 @@ ALWAYS_INLINE void PartitionPage::Free(void* ptr) {
   PartitionCookieCheckValue(ptr);
   PartitionCookieCheckValue(reinterpret_cast<char*>(ptr) + slot_size -
                             kCookieSize);
+#endif
 
+  // Perhaps surprisingly, this does not measurably regress performance. See
+  // https://crbug.com/680657 for history. We formerly did this in
+  // |PartitionFree|, and in `DCHECK_IS_ON()` builds we redundantly did it
+  // here, too. Now we only do it here, unconditionally.
   memset(ptr, kFreedByte, slot_size);
-#endif
 
   DCHECK(this->num_allocated_slots);
   // TODO(palmer): See if we can afford to make this a CHECK.