WebThreadSupportingGC: remove a stale comment
WebThreadSupportingGC: remove a stale comment

BUG=426332

Review URL: https://codereview.chromium.org/1292093002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@200529 bbb929c8-8fbe-4397-9dbb-9b2b20218538

Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk