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

src/heap/spaces.cc

Index: src/heap/spaces.cc
diff --git a/src/heap/spaces.cc b/src/heap/spaces.cc
index 7cfa4c294909fcb5bd3db25056d3947359983b2c..1ab5de737919cbe8d92fd9e0a31985144132aa7c 100644
--- a/src/heap/spaces.cc
+++ b/src/heap/spaces.cc
@@ -356,7 +356,8 @@ class MemoryAllocator::Unmapper::UnmapFreeMemoryTask : public v8::Task {
   // v8::Task overrides.
   void Run() override {
     unmapper_->PerformFreeMemoryOnQueuedChunks();
-    unmapper_->pending_unmapping_tasks_semaphore_.Signal();
+    unmapper_->pending_unmapping_tasks_semaphore_.Signal(
+        "Unmapper::UnmapFreeMemoryTask::Run");
   }
 
   Unmapper* unmapper_;