Developer Feature: Add Debug Accelerators to Toggle Touchscreen/Touchpad On or Off

Add two debugging accelerators for toggling the touchscreen / touchpad on or off.
These settings must be persistent between reboots. We ignore the case when users
turn off the debugging accelerators flag.

This is only for ChromeOS.
We add an UMA user metric to track how often they're used.

The two shortcuts are:
Search+Shift+T ---> Toggle touch screen.
Search+Shift+P ---> Toggle touchpad.


Review URL:

Cr-Commit-Position: refs/heads/master@{#358758}
39 files changed