Reland "[heap] Unprotect code pages on demand in MinorMC"

This reverts commit be2f237d.

Original change's description:
> [heap] Unprotect code pages on demand in MinorMC
>
> This reduces average pause of MinorMC in Richards benchmark from 0.32ms
> to 0.25ms:
>
> baseline pause
>   len: 22
>   min: 0.3
>   max: 0.6
>   avg: 0.322727272727
>   [0,5[: 22
>
> pause
>   len: 22
>   min: 0.2
>   max: 0.7
>   avg: 0.254545454545
>   [0,5[: 22
>
> Bug: chromium:651354

Change-Id: I9d70037dda612528368fb1ba330dc6f6510a14a6
Reviewed-on: https://chromium-review.googlesource.com/1055450Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53129}

src/heap/heap.cc

@@ -1884,7 +1884,6 @@ void Heap::MinorMarkCompact() {
   AlwaysAllocateScope always_allocate(isolate());
   IncrementalMarking::PauseBlackAllocationScope pause_black_allocation(
       incremental_marking());
-  CodeSpaceMemoryModificationScope code_modifcation(this);
   ConcurrentMarking::PauseScope pause_scope(concurrent_marking());
 
   minor_mark_compact_collector()->CollectGarbage();

src/heap/mark-compact.cc

@@ -270,11 +270,12 @@ void EvacuationVerifier::VerifyEvacuation(NewSpace* space) {
 void EvacuationVerifier::VerifyEvacuation(PagedSpace* space) {
   for (Page* p : *space) {
     if (p->IsEvacuationCandidate()) continue;
-    if (p->Contains(space->top()))
+    if (p->Contains(space->top())) {
+      CodePageMemoryModificationScope memory_modification_scope(p);
       heap_->CreateFillerObjectAt(
           space->top(), static_cast<int>(space->limit() - space->top()),
           ClearRecordedSlots::kNo);
-
+    }
     VerifyEvacuationOnPage(p->area_start(), p->area_end());
   }
 }
@@ -2763,6 +2764,7 @@ class RememberedSetUpdatingItem : public UpdatingItem {
     TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("v8.gc"),
                  "RememberedSetUpdatingItem::Process");
     base::LockGuard<base::Mutex> guard(chunk_->mutex());
+    CodePageMemoryModificationScope memory_modification_scope(chunk_);
     UpdateUntypedPointers();
     UpdateTypedPointers();
   }