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

Review URL: https://chromiumcodereview.appspot.com/12093008
Patch from Ben Noordhuis <info@bnoordhuis.nl>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13868 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

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();