DevTools: Fix inspector/extensions/extensions-panel.html after r172024

r172024 manifested a bug in WebInspector.isBeingEdited(), which implied
that an event target was always an element. This broke on events
forwarded from extensions, which are target-agnostic (so, a #document
could be an event.target).

TBR=vsevik

Review URL: https://codereview.chromium.org/245323002

git-svn-id: svn://svn.chromium.org/blink/trunk@172040 bbb929c8-8fbe-4397-9dbb-9b2b20218538