Bug 1281581 - save and restore errno in the profiler's signal handler; r=BenWa

The profiler's signal handler clobbers errno, via its calls to sem_post,
or via other functions that it transitively calls.  TSan complains about
this, as a sample arriving at the wrong time could make it look like a
function that failed actually succeeded, or vice versa.  Ensure that the
signal handler preserves the state of the world by saving and restoring
errno around its operation.

tools/profiler/core/platform-linux.cc

@@ -231,8 +231,12 @@ static void SetSampleContext(TickSample* sample, void* context)
 namespace {
 
 void ProfilerSignalHandler(int signal, siginfo_t* info, void* context) {
+  // Avoid TSan warning about clobbering errno.
+  int savedErrno = errno;
+
   if (!Sampler::GetActiveSampler()) {
     sem_post(&sSignalHandlingDone);
+    errno = savedErrno;
     return;
   }
 
@@ -253,6 +257,7 @@ void ProfilerSignalHandler(int signal, siginfo_t* info, void* context) {
 
   sCurrentThreadProfile = NULL;
   sem_post(&sSignalHandlingDone);
+  errno = savedErrno;
 }
 
 } // namespace