Bug 1016330 - (Part 1) Make CSS Coverage work with B2G; r=paul

https://bugzilla.mozilla.org/show_bug.cgi?id=1016330#c3
> prettifyCSS needs to be used client side or moved at the toolkit level.
Done

https://bugzilla.mozilla.org/show_bug.cgi?id=1016330#c4
> Move the call to makeRemote to DeveloperToolbar.jsm
Not done. See https://bugzilla.mozilla.org/show_bug.cgi?id=1016330#c6

https://bugzilla.mozilla.org/show_bug.cgi?id=1016330#c5
> We need to test first there's a cssusage actor.
> A toolbox might be debugging gecko < 32.
Done

https://bugzilla.mozilla.org/show_bug.cgi?id=1016330#c2
> Kill reference to this._tabActor.browser. This might create issues
> with sub documents ("load" is sent to browser even for inner documents.
> It's not the case with tabActor.window).
Future work.
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