Improve performance of proxy resolver by tracing DNS dependencies.

net/proxy/proxy_resolver_error_observer.h

Index: net/proxy/proxy_resolver_error_observer.h
diff --git a/net/proxy/proxy_resolver_error_observer.h b/net/proxy/proxy_resolver_error_observer.h
index 9176c2f233fb579a80aece21b211898232decee5..6ee9cbba1bb8dfed49a477a1dafbf7535d563ad8 100644
--- a/net/proxy/proxy_resolver_error_observer.h
+++ b/net/proxy/proxy_resolver_error_observer.h
@@ -11,10 +11,7 @@
 
 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() {}
@@ -23,6 +20,11 @@ class NET_EXPORT_PRIVATE 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: