[PartitionAlloc] Make pages inaccessible when decommitting

Change-Id: If8ffec8205961a2213dfe7150d374a828320c2c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2548104Reviewed-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}

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(),
-          PageKeepPermissionsIfPossible);
+          PageUpdatePermissions);
       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(),
-                                   PageKeepPermissionsIfPossible);
+                                   PageUpdatePermissions);
 
   // 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.