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.
Bug 1020222 - Remove unused l10n strings; r=mratcliffe
Bug 903418 - Remove unhelpful GCLI 'media reset' manual text; r=mratcliffe
Bug 896256 - Fix l10n for a number of strings in the GCLI 'cmd' command; r=mratcliffe
Bug 989752 - Fix strings in GCLI help command; r=mratcliffe
Merge fx-team to m-c. a=merge