Improve performance of proxy resolver by tracing DNS dependencies.

net/proxy/proxy_resolver_error_observer.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NET_PROXY_PROXY_RESOLVER_ERROR_OBSERVER_H_
 #define NET_PROXY_PROXY_RESOLVER_ERROR_OBSERVER_H_
 
 #include "base/basictypes.h"
 #include "base/string16.h"
 #include "net/base/net_export.h"
 
 namespace net {
 
-// Interface for observing JavaScript error messages from PAC scripts. The
-// default implementation of the ProxyResolverJSBindings takes a class
-// implementing this interface and forwards all JavaScript errors related to
-// PAC scripts.
+// Interface for observing JavaScript error messages from PAC scripts.
 class NET_EXPORT_PRIVATE ProxyResolverErrorObserver {
  public:
   ProxyResolverErrorObserver() {}
   virtual ~ProxyResolverErrorObserver() {}
 
   // Handler for when an error is encountered. |line_number| may be -1
   // if a line number is not applicable to this error. |error| is a message
   // describing the error.
+  //
+  // Note on threading: This may get called from a worker thread. If the
+  // backing proxy resolver is ProxyResolverV8Tracing, then it will not
+  // be called concurrently, however it will be called from a different
+  // thread than the proxy resolver's origin thread.
   virtual void OnPACScriptError(int line_number, const string16& error) = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ProxyResolverErrorObserver);
 };
 
 }  // namespace net
 
 #endif  // NET_PROXY_PROXY_RESOLVER_ERROR_OBSERVER_H_