Reduce CPU usage when no isolates need to be profiled.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 077e7745dff9e9e04af947878e194e542b90d72f..7c57111528a813c32c21e977b2900cdaff875af7 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -1004,15 +1004,24 @@ void Isolate::PrintJSON(JSONStream* stream, bool ref) {
 }
 
 
-void Isolate::ProfileInterrupt() {
+bool Isolate::ProfileInterrupt() {
+  if (profiler_data() == NULL) {
+    // Profiler not setup for isolate.
+    return false;
+  }
+  if (profiler_data()->blocked()) {
+    // Profiler blocked for this isolate.
+    return false;
+  }
   InterruptableThreadState* state = thread_state();
   if (state == NULL) {
     // Isolate is not scheduled on a thread.
     ProfileIdle();
-    return;
+    return true;
   }
   ASSERT(state->id != Thread::kInvalidThreadId);
   ThreadInterrupter::InterruptThread(state);
+  return true;
 }