v8: add %p option to --logfile, expands to pid

Useful when you are profiling multiple instances of V8 concurrently.
Submitted upstream: http://code.google.com/p/v8/issues/detail?id=2503

Refs #4617.

deps/v8/src/log-utils.cc

@@ -107,6 +107,9 @@ void Log::Initialize() {
                 // one character so we can escape the loop properly.
                 p--;
                 break;
+              case 'p':
+                stream.Add("%d", OS::GetCurrentProcessId());
+                break;
               case 't': {
                 // %t expands to the current time in milliseconds.
                 double time = OS::TimeCurrentMillis();