[PartitionAlloc] Revert "poison on free" behavior.

Performance regressions, alas.

Bug: 1005070, 1013329, 1013326, 1013324
Change-Id: I15c0a539d5adde6f86de98ceec301863708307f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1880145Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709322}

base/allocator/partition_allocator/partition_page.h

@@ -14,24 +14,6 @@
 #include "base/allocator/partition_allocator/random.h"
 #include "base/logging.h"
 
-namespace {
-
-// Returns true if we've hit the end of a random-length period. We don't want to
-// invoke `RandomValue` too often, because we call this function in a hot spot
-// (`Free`), and `RandomValue` incurs the cost of atomics.
-#if !DCHECK_IS_ON()
-bool RandomPeriod() {
-  static thread_local uint8_t counter = 0;
-  if (UNLIKELY(counter == 0)) {
-    counter = base::RandomValue();
-  }
-  counter--;
-  return counter == 0;
-}
-#endif
-
-}  // namespace
-
 namespace base {
 namespace internal {
 
@@ -220,24 +202,19 @@ ALWAYS_INLINE size_t PartitionPage::get_raw_size() const {
 }
 
 ALWAYS_INLINE void PartitionPage::Free(void* ptr) {
+#if DCHECK_IS_ON()
   size_t slot_size = this->bucket->slot_size;
   const size_t raw_size = get_raw_size();
   if (raw_size) {
     slot_size = raw_size;
   }
 
-#if DCHECK_IS_ON()
   // If these asserts fire, you probably corrupted memory.
   PartitionCookieCheckValue(ptr);
   PartitionCookieCheckValue(reinterpret_cast<char*>(ptr) + slot_size -
                             kCookieSize);
 
   memset(ptr, kFreedByte, slot_size);
-#else
-  // `memset` only once in a while.
-  if (UNLIKELY(RandomPeriod())) {
-    memset(ptr, kFreedByte, slot_size);
-  }
 #endif
 
   DCHECK(this->num_allocated_slots);