Revert "[PartitionAlloc] Make pages inaccessible when decommitting"

This reverts commit 53003343.

Reason for revert: Perf regressions
https://chromeperf.appspot.com/group_report?rev=832242

Original change's description:
> [PartitionAlloc] Make pages inaccessible when decommitting
>
> Change-Id: If8ffec8205961a2213dfe7150d374a828320c2c4
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2548104
> Reviewed-by: Kentaro Hara <haraken@chromium.org>
> Reviewed-by: Chris Palmer <palmer@chromium.org>
> Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#832242}

TBR=palmer@chromium.org,haraken@chromium.org,chromium-scoped@luci-project-accounts.iam.gserviceaccount.com,bartekn@chromium.org

Change-Id: Id673b2c3d9bbc19c98a44cb8a4780f99c0e7476b
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2567282
Auto-Submit: Bartek Nowierski <bartekn@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832364}

base/allocator/partition_allocator/partition_bucket.cc

@@ -569,7 +569,7 @@ void* PartitionBucket<thread_safe>::SlowPathAlloc(
       void* addr = SlotSpanMetadata<thread_safe>::ToPointer(new_slot_span);
       root->RecommitSystemPagesForData(
           addr, new_slot_span->bucket->get_bytes_per_span(),
-          PageUpdatePermissions);
+          PageKeepPermissionsIfPossible);
       new_slot_span->Reset();
       *is_already_zeroed = kDecommittedPagesAreAlwaysZeroed;
     }

base/allocator/partition_allocator/partition_page.cc

@@ -168,7 +168,7 @@ void SlotSpanMetadata<thread_safe>::Decommit(PartitionRoot<thread_safe>* root) {
   PA_DCHECK(!bucket->is_direct_mapped());
   void* addr = SlotSpanMetadata::ToPointer(this);
   root->DecommitSystemPagesForData(addr, bucket->get_bytes_per_span(),
-                                   PageUpdatePermissions);
+                                   PageKeepPermissionsIfPossible);
 
   // We actually leave the decommitted slot span in the active list. We'll sweep
   // it on to the decommitted list when we next walk the active list.