Really fix WebKit buildbot crashes on StopProfiling.

src/cpu-profiler.cc

Index: src/cpu-profiler.cc
diff --git a/src/cpu-profiler.cc b/src/cpu-profiler.cc
index d1230340e694c374a78642a489bb0216769e4238..234114908cedb3cdb7698365e7fcac070fa3b64e 100644
--- a/src/cpu-profiler.cc
+++ b/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;
 }