If the UI thread of the browser process gets so busy as to delaying the running of some tasks posted onto it, the user will feel temporary freeze of the browser UI. In some extreme cases, e.g. low memory on Chrome OS, there could be observable freezes (in seconds).

We may consider implementing a jank detector to help us find out what blocks the UI thread and delays the execution of UI thread tasks. One way to implement it is hooking the detector to the message loop for recording task execution data, including but not limited to:

* task origin (the program counter of FROM_HERE), and possibly the origin of its parent tasks.
* task timing (time when the task is posted, starts and finishes running).

and log the data of the "blocker" tasks if they delay the execution of some task too much.