Instrument callers of Semaphore::Signal to help with investigation of

src/heap/mark-compact.cc

Index: src/heap/mark-compact.cc
diff --git a/src/heap/mark-compact.cc b/src/heap/mark-compact.cc
index 89ca782c267b317fc64bcb27f4c9442bb6424a7d..ad0c58e4a8d53e61c5c459caec33d9299184d065 100644
--- a/src/heap/mark-compact.cc
+++ b/src/heap/mark-compact.cc
@@ -480,7 +480,7 @@ class MarkCompactCollector::Sweeper::SweeperTask : public v8::Task {
       DCHECK_LE(space_id, LAST_PAGED_SPACE);
       sweeper_->ParallelSweepSpace(static_cast<AllocationSpace>(space_id), 0);
     }
-    pending_sweeper_tasks_->Signal();
+    pending_sweeper_tasks_->Signal("SweeperTask::Run");
   }
 
   Sweeper* sweeper_;
@@ -585,7 +585,7 @@ bool MarkCompactCollector::Sweeper::IsSweepingCompleted() {
           base::TimeDelta::FromSeconds(0))) {
     return false;
   }
-  pending_sweeper_tasks_semaphore_.Signal();
+  pending_sweeper_tasks_semaphore_.Signal("Sweeper::IsSweepingCompleted");
   return true;
 }