simplify utility to hit-test focused plugin

After about 4 taps, this starts to return that the tap was
in the plugin. Before that, the plugin doesn't have focus.
Derek may want to verify that everything is working as it
should -- maybe it just takes that long for webkit to take
focus.

companion fix in external/webkit

Change-Id: I80ffba6c55876b0af9c305c539a10e8e8ed7cb3c
http://b/2521087