Provide a counter for thrown JavaScript errors per context

src/isolate.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/isolate.h"
 
 #include <stdlib.h>
 
 #include <fstream>  // NOLINT(readability/streams)
 #include <sstream>
@@ skipping 976 matching lines @@
   bool requires_message = try_catch_handler() == nullptr ||
                           try_catch_handler()->is_verbose_ ||
                           try_catch_handler()->capture_message_;
   bool rethrowing_message = thread_local_top()->rethrowing_message_;
 
   thread_local_top()->rethrowing_message_ = false;
 
   // 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.
   if (requires_message && !rethrowing_message) {
     MessageLocation computed_location;
     // If no location was specified we try to use a computed one instead.
     if (location == NULL && ComputeLocation(&computed_location)) {
       location = &computed_location;
     }
 
@@ skipping 1836 matching lines @@
   // Then check whether this scope intercepts.
   if ((flag & intercept_mask_)) {
     intercepted_flags_ |= flag;
     return true;
   }
   return false;
 }
 
 }  // namespace internal
 }  // namespace v8