build: Upgrade karma-chrome-launcher to support Chromium on Mac/Windows

build: Upgrade karma-chrome-launcher to support Chromium on Mac/Windows

Before karma-chrome-launcher 2.0.0, Chromium was only detected on Linux
and as part of the "Chrome" setting. "Chromium" is now recognised as a
separate browser and is detected on Mac and Windows as well.

https://github.com/karma-runner/karma-chrome-launcher/issues/45

Add a grunt target for Chromium so users may run the following to use
Chromium instead of Chrome:

    grunt karma:chromium

Since we keep the default of "Chrome", we'll need an override for CI
since we install Chromium there instead of Chrome.

Depends on Ie9f906f8 being merged and Nodepool images updated.

Bug: T153756
Change-Id: I6ba0d971e7e8c3022d2ab6268ae637688189d768