Add chromeHeadless browser option to use headless chromium easily

Currently, the only way to use headless chromium is to use chrome as a browser and then add --headless as a chromeArguments property:

<property name="browser">chrome</property>
<property name="chromeArguments">--headless</property>

The idea of this task is making it simpler - just using chromeHeadless as a browser property:

<property name="browser">chromeHeadless</property>