Merge r7549 "Fix auto suspension of the sampler thread" to trunk.

src/isolate.h

Index: src/isolate.h
===================================================================
--- src/isolate.h	(revision 7548)
+++ src/isolate.h	(working copy)
@@ -897,13 +897,19 @@
 
   void SetCurrentVMState(StateTag state) {
     if (RuntimeProfiler::IsEnabled()) {
-      if (state == JS) {
-        // JS or non-JS -> JS transition.
+      StateTag current_state = thread_local_top_.current_vm_state_;
+      if (current_state != JS && state == JS) {
+        // Non-JS -> JS transition.
         RuntimeProfiler::IsolateEnteredJS(this);
-      } else if (thread_local_top_.current_vm_state_ == JS) {
+      } else if (current_state == JS && state != JS) {
         // JS -> non-JS transition.
         ASSERT(RuntimeProfiler::IsSomeIsolateInJS());
         RuntimeProfiler::IsolateExitedJS(this);
+      } else {
+        // Other types of state transitions are not interesting to the
+        // runtime profiler, because they don't affect whether we're
+        // in JS or not.
+        ASSERT((current_state == JS) == (state == JS));
       }
     }
     thread_local_top_.current_vm_state_ = state;