Oilpan: fix webkit unit tests after r194688.
Oilpan: fix webkit unit tests after r194688.

When a plugin element is detached, Oilpan dispose()s of the plugin
container to clear out required references. Take that into account
when checking for detached state in isRectTopmost().

TBR=oilpan-reviews
BUG=476590
NOTRY=true

Review URL: https://codereview.chromium.org/1112953003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@194703 bbb929c8-8fbe-4397-9dbb-9b2b20218538

XiaosongWei/blink-crosswalk,XiaosongWei/blink-crosswalk,XiaosongWei/blink-crosswalk,PeterWangIntel/blink-crosswalk,Bysmyyr/blink-crosswalk,Bysmyyr/blink-crosswalk,smishenk/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,XiaosongWei/blink-crosswalk,Bysmyyr/blink-crosswalk,Pluto-tv/blink-crosswalk,XiaosongWei/blink-crosswalk,XiaosongWei/blink-crosswalk,PeterWangIntel/blink-crosswalk,Pluto-tv/blink-crosswalk,PeterWangIntel/blink-crosswalk,Bysmyyr/blink-crosswalk,Pluto-tv/blink-crosswalk,PeterWangIntel/blink-crosswalk,Pluto-tv/blink-crosswalk,PeterWangIntel/blink-crosswalk,PeterWangIntel/blink-crosswalk,smishenk/blink-crosswalk,XiaosongWei/blink-crosswalk,Bysmyyr/blink-crosswalk,Bysmyyr/blink-crosswalk,smishenk/blink-crosswalk,XiaosongWei/blink-crosswalk,Bysmyyr/blink-crosswalk,Bysmyyr/blink-crosswalk,Pluto-tv/blink-crosswalk,smishenk/blink-crosswalk,smishenk/blink-crosswalk,smishenk/blink-crosswalk,PeterWangIntel/blink-crosswalk,Bysmyyr/blink-crosswalk,Bysmyyr/blink-crosswalk,smishenk/blink-crosswalk,PeterWangIntel/blink-crosswalk,XiaosongWei/blink-crosswalk,PeterWangIntel/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,XiaosongWei/blink-crosswalk,PeterWangIntel/blink-crosswalk,smishenk/blink-crosswalk,smishenk/blink-crosswalk,smishenk/blink-crosswalk,Pluto-tv/blink-crosswalk