This option helps avoiding blank inspector situations when trying to use the right-click ctx
menu in the browser to inspect an element *while* the page is reloading.
helper_outerhtml_test_runner.js was accessing DOM nodes directly in content (via
CPOWs) to check their outerHTML.
This change adds outerHTML, innerHTML and textContent to the common
devtools:test:getDomElementInfo frame-script message listener so that the test
can get it there instead.
This change loads the devtools common frame-script-utils.js frame-script in the browser message
manager when a new test tab is opened by a test, and it adds a new getDomElementInfo message
listener useful for many tests to retrieve data about a node without having to go through a
CPOW.
