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

src/log-utils.cc

Index: src/log-utils.cc
diff --git a/src/log-utils.cc b/src/log-utils.cc
index d8d92cbe211607f03e2a6f131f6f2f481b47f023..a66db3d931e9fe5d99edfba39b81ec8418099a89 100644
--- a/src/log-utils.cc
+++ b/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();