Add tracing events to track sync ipc blocker

ipc/ipc_sync_channel.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef IPC_IPC_SYNC_CHANNEL_H_
 #define IPC_IPC_SYNC_CHANNEL_H_
 
 #include <deque>
 #include <string>
 #include <vector>
@@ skipping 138 matching lines @@
     // needs to get dispatched (by calling SyncContext::DispatchMessages).
     base::WaitableEvent* GetDispatchEvent();
 
     void DispatchMessages();
 
     // Checks if the given message is blocking the listener thread because of a
     // synchronous send.  If it is, the thread is unblocked and true is
     // 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() {
       return received_sync_msgs_.get();
     }
 
     void set_restrict_dispatch_group(int group) {
       restrict_dispatch_group_ = group;
     }
 
@@ skipping 66 matching lines @@
 
   // Tracks SyncMessageFilters created before complete channel initialization.
   std::vector<scoped_refptr<SyncMessageFilter>> pre_init_sync_message_filters_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncChannel);
 };
 
 }  // namespace IPC
 
 #endif  // IPC_IPC_SYNC_CHANNEL_H_