Provide a counter for thrown JavaScript errors per context

src/isolate.cc

Index: src/isolate.cc
diff --git a/src/isolate.cc b/src/isolate.cc
index 64a5c05b5a38d4706469776fa9f1c22142bccc95..8a2c1937d5adcc410b501817862086b74f0f8d26 100644
--- a/src/isolate.cc
+++ b/src/isolate.cc
@@ -994,6 +994,7 @@ Object* Isolate::Throw(Object* exception, MessageLocation* location) {
   // Notify debugger of exception.
   if (is_catchable_by_javascript(exception)) {
     debug()->OnThrow(exception_handle);
+    this->counters()->exceptions_thrown()->Increment();

[Yang, 2015/10/23 04:53:20]

You can omit "this->"

Putting the counter increment here would include every thrown exception, not
just uncaught ones.

The problem here is that throw is just normal Javascript syntax. Whether or how
people use it in content scripts is entirely their choice. For example, you can
(ab)use it for control flow, it's a perfectly valid and acceptable way to reject
a Promise.

If we decide to count all exceptions, maybe we can limit that to ones thrown by
V8? A possible place to add the counter increment for this purpose is
v8::internal::Runtime_FormatMessage.

   }
 
   // Generate the message if required.