[Ozone-Drm] Do not try to take ref_ptr from destructor
[Ozone-Drm] Do not try to take ref_ptr from destructor

Can't post a task with a ref-counted instance of the watcher from the
destructor.

BUG=467170

Review URL: https://codereview.chromium.org/1006403002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#320739}

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