Revert "[heap] Clear from space after garbage collection."

This reverts commit 494068c1.

Reason for revert: https://bugs.chromium.org/p/chromium/issues/detail?id=840282

Original change's description:
> [heap] Clear from space after garbage collection.
>
> Bug: chromium:829771
> Change-Id: I9e71e6cbba347dd6951e5415332e5178df9b5122
> Reviewed-on: https://chromium-review.googlesource.com/1041685
> Commit-Queue: Hannes Payer <hpayer@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#52957}

TBR=hpayer@chromium.org,mlippautz@chromium.org

Bug: chromium:829771
Change-Id: I3c36baaf849a7d5dbf48db2ae2178e15b3b886f8
Reviewed-on: https://chromium-review.googlesource.com/1049888
Commit-Queue: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53079}

src/heap/heap.cc

@@ -920,7 +920,10 @@ void Heap::DeoptMarkedAllocationSites() {
 
 void Heap::GarbageCollectionEpilogue() {
   TRACE_GC(tracer(), GCTracer::Scope::HEAP_EPILOGUE);
-  ZapFromSpace();
+  // In release mode, we only zap the from space under heap verification.
+  if (Heap::ShouldZapGarbage()) {
+    ZapFromSpace();
+  }
 
 #ifdef VERIFY_HEAP
   if (FLAG_verify_heap) {
@@ -3837,13 +3840,12 @@ void Heap::VerifyCountersBeforeConcurrentSweeping() {
 
 void Heap::ZapFromSpace() {
   if (!new_space_->IsFromSpaceCommitted()) return;
-
   for (Page* page :
        PageRange(new_space_->FromSpaceStart(), new_space_->FromSpaceEnd())) {
-    memory_allocator()->ZapBlock(page->area_start(), page->area_size(),
-                                 Heap::ShouldZapGarbage()
-                                     ? kFromSpaceZapValue
-                                     : kClearedFreeMemoryValue);
+    for (Address cursor = page->area_start(), limit = page->area_end();
+         cursor < limit; cursor += kPointerSize) {
+      Memory::Address_at(cursor) = static_cast<Address>(kFromSpaceZapValue);
+    }
   }
 }