Make Menu.findMenuItem_ return the first MenuItem.
Make Menu.findMenuItem_ return the first MenuItem.

This fixes the regression in http://crbug.com/480381 where 'activate'
event is not fired for menu items under a grouping div rather than
being direct children of a Menu. Also fixes js error when hovering
such menu items.

BUG=480381

Review URL: https://codereview.chromium.org/1095303004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#327080}

PeterWangIntel/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,Fireblend/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,PeterWangIntel/chromium-crosswalk,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,Fireblend/chromium-crosswalk,chuan9/chromium-crosswalk,hgl888/chromium-crosswalk,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Just-D/chromium-1,hgl888/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,axinging/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Chilledheart/chromium,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,hgl888/chromium-crosswalk,chuan9/chromium-crosswalk,Pluto-tv/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,hgl888/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,Pluto-tv/chromium-crosswalk,Just-D/chromium-1,Chilledheart/chromium,TheTypoMaster/chromium-crosswalk,Chilledheart/chromium,Chilledheart/chromium,Just-D/chromium-1,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Chilledheart/chromium,Pluto-tv/chromium-crosswalk,chuan9/chromium-crosswalk,hgl888/chromium-crosswalk,chuan9/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Fireblend/chromium-crosswalk,axinging/chromium-crosswalk,hgl888/chromium-crosswalk,Fireblend/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Fireblend/chromium-crosswalk,chuan9/chromium-crosswalk,Just-D/chromium-1,hgl888/chromium-crosswalk,Fireblend/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,chuan9/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,Chilledheart/chromium,Pluto-tv/chromium-crosswalk,Just-D/chromium-1,chuan9/chromium-crosswalk,chuan9/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,PeterWangIntel/chromium-crosswalk,Fireblend/chromium-crosswalk,Fireblend/chromium-crosswalk,chuan9/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Fireblend/chromium-crosswalk