Really fix WebKit buildbot crashes on StopProfiling.

This time I hope I'm doing the right check.

TBR=ager@chromium.org

Review URL: http://codereview.chromium.org/1630027

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4439 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/cpu-profiler.cc

@@ -253,12 +253,12 @@ void CpuProfiler::StartProfiling(String* 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;
 }