Add tracing events to track sync ipc blocker

ipc/ipc_sync_message_filter.cc

Index: ipc/ipc_sync_message_filter.cc
diff --git a/ipc/ipc_sync_message_filter.cc b/ipc/ipc_sync_message_filter.cc
index 1a6f6d9255a723d2a98bb938e3d0331d2e7885d9..73e905833ed34f675d35e355affd63c2cdef30b4 100644
--- a/ipc/ipc_sync_message_filter.cc
+++ b/ipc/ipc_sync_message_filter.cc
@@ -59,7 +59,10 @@ bool SyncMessageFilter::Send(Message* message) {
   }
 
   base::WaitableEvent* events[2] = { shutdown_event_, &done_event };
-  base::WaitableEvent::WaitMany(events, 2);
+  if (base::WaitableEvent::WaitMany(events, 2) == 1) {
+    TRACE_EVENT_FLOW_END0(TRACE_DISABLED_BY_DEFAULT("ipc.flow"),
+                          "SyncMessageFilter::Send", &done_event);
+  }
 
   {
     base::AutoLock auto_lock(lock_);
@@ -103,6 +106,9 @@ bool SyncMessageFilter::OnMessageReceived(const Message& message) {
         (*iter)->send_result =
             (*iter)->deserializer->SerializeOutputParameters(message);
       }
+      TRACE_EVENT_FLOW_BEGIN0(TRACE_DISABLED_BY_DEFAULT("ipc.flow"),
+                              "SyncMessageFilter::OnMessageReceived",
+                              (*iter)->done_event);
       (*iter)->done_event->Signal();
       return true;
     }
@@ -142,6 +148,9 @@ void SyncMessageFilter::SignalAllEvents() {
   lock_.AssertAcquired();
   for (PendingSyncMessages::iterator iter = pending_sync_messages_.begin();
        iter != pending_sync_messages_.end(); ++iter) {
+    TRACE_EVENT_FLOW_BEGIN0(TRACE_DISABLED_BY_DEFAULT("ipc.flow"),
+                            "SyncMessageFilter::SignalAllEvents",
+                            (*iter)->done_event);
     (*iter)->done_event->Signal();
   }
 }