[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: I701ca800d7c6986534d1de2e3051476e91a88d7d
Reviewed-on: https://chromium-review.googlesource.com/1055507Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53125}

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

@@ -2810,6 +2810,7 @@ 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();
   }