r173929 changed that web inspector runs on a separate web process. Since that, EFL inspector begun not to work correctly. r174820 fixed that web inspector isn't shown though, highlight function doesn't still work.
I experimented with it a little bit, and the root of the problem is http://trac.webkit.org/changeset/174231 , not r173929. It seems the Inspector Overlay isn't shown at all after r174231 and its workaround r174435.
GTK fixed a similar bug: bug131433 , maybe it helps fixing this bug.
Closing this bug because the EFL port has been removed from trunk. If you feel this bug applies to a different upstream WebKit port and was closed in error, please either update the title and reopen the bug, or leave a comment to request this.