Really fix WebKit buildbot crashes on StopProfiling.

src/cpu-profiler.cc

 // Copyright 2010 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 235 matching lines @@
 }
 
 
 void CpuProfiler::StartProfiling(String* title) {
   ASSERT(singleton_ != NULL);
   singleton_->StartCollectingProfile(title);
 }
 
 
 CpuProfile* CpuProfiler::StopProfiling(const char* title) {
-  return singleton_ != NULL ? singleton_->StopCollectingProfile(title) : NULL;
+  return is_profiling() ? singleton_->StopCollectingProfile(title) : NULL;
 }
 
 
 CpuProfile* CpuProfiler::StopProfiling(String* title) {
-  return singleton_ != NULL ? singleton_->StopCollectingProfile(title) : NULL;
+  return is_profiling() ? singleton_->StopCollectingProfile(title) : NULL;
 }
 
 
 int CpuProfiler::GetProfilesCount() {
   ASSERT(singleton_ != NULL);
   return singleton_->profiles_->profiles()->length();
 }
 
 
 CpuProfile* CpuProfiler::GetProfile(int index) {
@@ skipping 211 matching lines @@
 void CpuProfiler::TearDown() {
 #ifdef ENABLE_LOGGING_AND_PROFILING
   if (singleton_ != NULL) {
     delete singleton_;
   }
   singleton_ = NULL;
 #endif
 }
 
 } }  // namespace v8::internal