Fix bluetooth crash after hotrod enrollment.
Fix bluetooth crash after hotrod enrollment.

OnSocketDeactivate is called on the ui thread, so calling Thread::Stop is inappropriate (and triggers a DCHECK).
Cleanup of the thread and task runner occur when BluetoothSocketThread is deleted.

BUG=503246

Review URL: https://codereview.chromium.org/1191203004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#336303}

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