[heap] Remove unused flag prohibiting local slot buffer entries

This is a fix that removes a left over from
  https://crrev.com/4566531c6e593ae8bffc2ef96f82c91d15a68acf

With this flag set to false still present, we would race on inserting in the
local old-to-new remembered set.

R=ulan@chormium.org
BUG=chromium:524425
LOG=N

Review URL: https://codereview.chromium.org/1783283003

Cr-Commit-Position: refs/heads/master@{#34728}

src/heap/mark-compact.cc

@@ -62,7 +62,6 @@ MarkCompactCollector::MarkCompactCollector(Heap* heap)
       have_code_to_deoptimize_(false),
       compacting_(false),
       sweeping_in_progress_(false),
-      compaction_in_progress_(false),
       pending_sweeper_tasks_semaphore_(0),
       pending_compaction_tasks_semaphore_(0) {
 }
@@ -2512,12 +2511,7 @@ void MarkCompactCollector::RecordMigratedSlot(
   // When parallel compaction is in progress, store and slots buffer entries
   // require synchronization.
   if (heap_->InNewSpace(value)) {
-    if (compaction_in_progress_) {
-      old_to_new_slots->Record(slot);
-    } else {
-      Page* page = Page::FromAddress(slot);
-      RememberedSet<OLD_TO_NEW>::Insert(page, slot);
-    }
+    old_to_new_slots->Record(slot);
   } else if (value->IsHeapObject() && IsOnEvacuationCandidate(value)) {
     old_to_old_slots->Record(slot);
   }

src/heap/mark-compact.h

@@ -767,9 +767,6 @@ class MarkCompactCollector {
   // True if concurrent or parallel sweeping is currently in progress.
   bool sweeping_in_progress_;
 
-  // True if parallel compaction is currently in progress.
-  bool compaction_in_progress_;
-
   // Semaphore used to synchronize sweeper tasks.
   base::Semaphore pending_sweeper_tasks_semaphore_;