Add tracing events to track sync ipc blocker

ipc/ipc_sync_channel.h

Index: ipc/ipc_sync_channel.h
diff --git a/ipc/ipc_sync_channel.h b/ipc/ipc_sync_channel.h
index 240a28efc98deb0f69a0966534cfba1b6e98e21a..eb2a272330cb28d67de55b8e21644d4d55de21cc 100644
--- a/ipc/ipc_sync_channel.h
+++ b/ipc/ipc_sync_channel.h
@@ -156,10 +156,6 @@ class IPC_EXPORT SyncChannel : public ChannelProxy {
     // returned. Otherwise the function returns false.
     bool TryToUnblockListener(const Message* msg);
 
-    // Called on the IPC thread when a sync send that runs a nested message loop

[Tom Sepez, 2016/03/29 16:14:38]

Why did we remove this?

[tzik, 2016/03/29 18:09:23]

Because no one uses this and this function touches |done_event|.
I'd like to remove this if it's acceptable rather than adding a tracing point to
the unused function.

-    // times out.
-    void OnSendTimeout(int message_id);
-
     base::WaitableEvent* shutdown_event() { return shutdown_event_; }
 
     ReceivedSyncMsgQueue* received_sync_msgs() {