[heap] Remove unused Sweeper::AddSweepingPageSafe.

BUG=v8:6474

Change-Id: I820ed47f18a41a1e281a78f1754f483a81627de8
Reviewed-on: https://chromium-review.googlesource.com/528104Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45854}

src/heap/mark-compact.cc

@@ -4684,12 +4684,6 @@ Page* MarkCompactCollector::Sweeper::GetSweepingPageSafe(
   return page;
 }
 
-void MarkCompactCollector::Sweeper::AddSweepingPageSafe(AllocationSpace space,
-                                                        Page* page) {
-  base::LockGuard<base::Mutex> guard(&mutex_);
-  sweeping_list_[space].push_back(page);
-}
-
 void MarkCompactCollector::StartSweepSpace(PagedSpace* space) {
   space->ClearStats();
 

src/heap/mark-compact.h

@@ -414,7 +414,6 @@ class MarkCompactCollector final : public MarkCompactCollectorBase {
     }
 
     Page* GetSweepingPageSafe(AllocationSpace space);
-    void AddSweepingPageSafe(AllocationSpace space, Page* page);
 
     void PrepareToBeSweptPage(AllocationSpace space, Page* page);