Provide a counter for thrown JavaScript errors per context

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index 0b63ceb8f980bd336b027ba46b5c067a656a19d6..9cf9253ad4ee00fd70e109764abea6f6bedf7e45 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -373,6 +373,10 @@ static void SetObjectPrototype(Handle<JSObject> object, Handle<Object> proto) {
 
 
 void Bootstrapper::DetachGlobal(Handle<Context> env) {
+  // add thrown javascript exceptions
+  env->GetIsolate()->counters()->exceptions_thrown_per_context()->AddSample(
+    env->GetExceptionsThrown());
+
   Factory* factory = env->GetIsolate()->factory();
   Handle<JSGlobalProxy> global_proxy(JSGlobalProxy::cast(env->global_proxy()));
   global_proxy->set_native_context(*factory->null_value());