Remove sequential sweeping mode and perform lazy sweeping when no sweeper threads are active.

src/spaces.h

Index: src/spaces.h
diff --git a/src/spaces.h b/src/spaces.h
index 855359835293d05bf7523a7a5b187d96a4ec7cbb..32850781452f82b1fe880a9ef2a8d3bd0a017fba 100644
--- a/src/spaces.h
+++ b/src/spaces.h
@@ -1905,9 +1905,6 @@ class PagedSpace : public Space {
   bool is_iterable() { return is_iterable_; }
   void set_is_iterable(bool b) { is_iterable_ = b; }
 
-  bool is_swept_concurrently() { return is_swept_concurrently_; }
-  void set_is_swept_concurrently(bool b) { is_swept_concurrently_ = b; }
-
   // Evacuation candidates are swept by evacuator.  Needs to return a valid
   // result before _and_ after evacuation has finished.
   static bool ShouldBeSweptBySweeperThreads(Page* p) {
@@ -1992,9 +1989,6 @@ class PagedSpace : public Space {
   // This space was swept precisely, hence it is iterable.
   bool is_iterable_;
 
-  // This space is currently swept by sweeper threads.
-  bool is_swept_concurrently_;
-
   // The number of free bytes which could be reclaimed by advancing the
   // concurrent sweeper threads.  This is only an estimation because concurrent
   // sweeping is done conservatively.
@@ -2017,7 +2011,8 @@ class PagedSpace : public Space {
   // If sweeping is still in progress try to sweep unswept pages. If that is
   // not successful, wait for the sweeper threads and re-try free-list
   // allocation.
-  MUST_USE_RESULT HeapObject* EnsureSweepingProgress(int size_in_bytes);
+  MUST_USE_RESULT HeapObject* WaitForSweeperThreadsAndRetryAllocation(
+      int size_in_bytes);
 
   // Slow path of AllocateRaw.  This function is space-dependent.
   MUST_USE_RESULT HeapObject* SlowAllocateRaw(int size_in_bytes);