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

src/base/platform/semaphore.h

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_BASE_PLATFORM_SEMAPHORE_H_
 #define V8_BASE_PLATFORM_SEMAPHORE_H_
 
 #include "src/base/lazy-instance.h"
 #if V8_OS_WIN
 #include "src/base/win32-headers.h"
@@ skipping 19 matching lines @@
 // object and incremented each time a thread signals the semaphore. When the
 // count reaches zero,  threads waiting for the semaphore blocks until the
 // count becomes non-zero.
 
 class Semaphore final {
  public:
   explicit Semaphore(int count);
   ~Semaphore();
 
   // Increments the semaphore counter.
-  void Signal();
+  // TODO(ulan): remove caller parameter once crbug.com/609249 is fixed.
+  void Signal(const char* caller = nullptr);
 
   // Suspends the calling thread until the semaphore counter is non zero
   // and then decrements the semaphore counter.
   void Wait();
 
   // Suspends the calling thread until the counter is non zero or the timeout
   // time has passed. If timeout happens the return value is false and the
   // counter is unchanged. Otherwise the semaphore counter is decremented and
   // true is returned.
   bool WaitFor(const TimeDelta& rel_time) WARN_UNUSED_RESULT;
@@ skipping 42 matching lines @@
   typedef typename LazyDynamicInstance<Semaphore, CreateSemaphoreTrait<N>,
                                        ThreadSafeInitOnceTrait>::type type;
 };
 
 #define LAZY_SEMAPHORE_INITIALIZER LAZY_DYNAMIC_INSTANCE_INITIALIZER
 
 }  // namespace base
 }  // namespace v8
 
 #endif  // V8_BASE_PLATFORM_SEMAPHORE_H_