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 66af826b5bb8e169f86d84f5e41a5ce068f50f0f..55a43966d163d28a7efbaf54dad256562579caa8 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -871,13 +871,15 @@ class Isolate {
 #undef NATIVE_CONTEXT_FIELD_ACCESSOR
 
   Bootstrapper* bootstrapper() { return bootstrapper_; }
-  Counters* counters() {
-    // Call InitializeLoggingAndCounters() if logging is needed before
-    // the isolate is fully initialized.
-    DCHECK_NOT_NULL(counters_shared_.get());
-    return counters_shared_.get();
+  // Use for updating counters on a foreground thread.
+  Counters* counters() { return async_counters().get(); }
+  // Use for updating counters on a background thread.
+  const std::shared_ptr<Counters>& async_counters() {
+    DCHECK(!IsIsolateInBackground());
+    // Make sure InitializeCounters() has been called.
+    DCHECK_NOT_NULL(async_counters_.get());
+    return async_counters_;
   }
-  std::shared_ptr<Counters> counters_shared() { return counters_shared_; }
   RuntimeProfiler* runtime_profiler() { return runtime_profiler_; }
   CompilationCache* compilation_cache() { return compilation_cache_; }
   Logger* logger() {
@@ -1443,7 +1445,7 @@ class Isolate {
   Bootstrapper* bootstrapper_;
   RuntimeProfiler* runtime_profiler_;
   CompilationCache* compilation_cache_;
-  std::shared_ptr<Counters> counters_shared_;
+  std::shared_ptr<Counters> async_counters_;
   base::RecursiveMutex break_access_;
   Logger* logger_;
   StackGuard stack_guard_;