PSA: A fix for web-exposed First Input Delay

[Hongbo Song]

Hi all,

Recently we landed a CL for web-exposed First Input Delay. 

On the desktop, when we click on a web page, the input event sequence would be pointerdown, mousedown, pointerup, mouseup. The first input should be the pointerdown. But our computation of FID used to regard the mousedown as the first input on the desktop. In the CL, we wouldn't report the mousedown as the FID if we already have a pointerdown.

This change is just a spec alignment bug fix.

Thanks!