Fix use of history timers in background threads.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index aad853dc0851fe670179a8fab4ebe720744e40a0..7a376b1a3a3003f385d7513a9a7bb814d1bae6ce 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -866,13 +866,20 @@ class Isolate {
 #undef NATIVE_CONTEXT_FIELD_ACCESSOR
 
   Bootstrapper* bootstrapper() { return bootstrapper_; }
+  // Use for updating counters on a foreground thread.
   Counters* counters() {

[jochen (gone - plz use gerrit), 2017/06/12 09:18:34]

why not have counters() always return the shared_ptr?

[kschimpf, 2017/06/12 16:47:05]

Because too much code accesses it like this:

   Counters* counters = isolate->counter();

and then reference 'counters' multiple times.

I did not want this CL to get (very) large.

-    // Call InitializeLoggingAndCounters() if logging is needed before
-    // the isolate is fully initialized.
+    DCHECK(!IsIsolateInBackground());
+    // Make sure InitializeCounters() has been called.
     DCHECK_NOT_NULL(counters_shared_.get());
     return counters_shared_.get();
   }
-  std::shared_ptr<Counters> counters_shared() { return counters_shared_; }
+  // Use for updating counters on a background thread.
+  std::shared_ptr<Counters> counters_shared() {
+    DCHECK(!IsIsolateInBackground());
+    // Make sure InitializeCounters() has been called.
+    DCHECK_NOT_NULL(counters_shared_.get());
+    return counters_shared_;
+  }
   RuntimeProfiler* runtime_profiler() { return runtime_profiler_; }
   CompilationCache* compilation_cache() { return compilation_cache_; }
   Logger* logger() {