Use the location where FileDescriptorWatcher is created to track the libevent handlers

mojo/edk/system/channel_posix.cc

Index: mojo/edk/system/channel_posix.cc
diff --git a/mojo/edk/system/channel_posix.cc b/mojo/edk/system/channel_posix.cc
index f4687562800c3d2d51c54c85ffa0d800b7454084..fb43b85d877a944dd50843ed2c8bea5d9431b1ab 100644
--- a/mojo/edk/system/channel_posix.cc
+++ b/mojo/edk/system/channel_posix.cc
@@ -210,14 +210,16 @@ class ChannelPosix : public Channel,
   void StartOnIOThread() {
     DCHECK(!read_watcher_);
     DCHECK(!write_watcher_);
-    read_watcher_.reset(new base::MessageLoopForIO::FileDescriptorWatcher);
+    read_watcher_.reset(
+        new base::MessageLoopForIO::FileDescriptorWatcher(FROM_HERE));
     base::MessageLoop::current()->AddDestructionObserver(this);
     if (handle_.get().needs_connection) {
       base::MessageLoopForIO::current()->WatchFileDescriptor(
           handle_.get().handle, false /* persistent */,
           base::MessageLoopForIO::WATCH_READ, read_watcher_.get(), this);
     } else {
-      write_watcher_.reset(new base::MessageLoopForIO::FileDescriptorWatcher);
+      write_watcher_.reset(
+          new base::MessageLoopForIO::FileDescriptorWatcher(FROM_HERE));
       base::MessageLoopForIO::current()->WatchFileDescriptor(
           handle_.get().handle, true /* persistent */,
           base::MessageLoopForIO::WATCH_READ, read_watcher_.get(), this);