Bug 1080344 - [Keyboard] Take into account bounding client rect to handle difference in test-runner resolutions

https://bugzilla.mozilla.org/show_bug.cgi?id=1080344

https://treeherder.allizom.org/ui/#/jobs?repo=gaia&revision=6c05a632d07e7de62f694495ef4c7310fd1e2109