Null check StyleEngine in ~ShadowRoot.
Null check StyleEngine in ~ShadowRoot.

Not all documents have a StyleEngine, only displayed (active) ones.
We need a null check to avoid crashing in the ShadowRoot destructor.

BUG=443423
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/814713002

jason-simmons/sky_engine,mpcomplete/flutter_engine,takaaptech/sky_engine,zhangxq5012/sky_engine,mxia/engine,devoncarew/sky_engine,krisgiesing/sky_engine,lyceel/engine,flutter/engine,jason-simmons/flutter_engine,chinmaygarde/sky_engine,mdakin/engine,abarth/sky_engine,devoncarew/engine,devoncarew/engine,jason-simmons/flutter_engine,tvolkert/engine,aam/engine,mikejurka/engine,cdotstout/sky_engine,iansf/sky_engine,devoncarew/engine,qiankunshe/sky_engine,aam/engine,xunmengfeng/engine,afandria/sky_engine,zhangxq5012/sky_engine,jamesr/sky_engine,krisgiesing/sky_engine,aam/engine,mpcomplete/flutter_engine,tvolkert/engine,takaaptech/sky_engine,flutter/engine,jimsimon/sky_engine,mpcomplete/engine,chinmaygarde/flutter_engine,axinging/sky_engine,jimsimon/sky_engine,chinmaygarde/sky_engine,devoncarew/engine,xunmengfeng/engine,xunmengfeng/engine,jason-simmons/sky_engine,afandria/sky_engine,chinmaygarde/flutter_engine,afandria/sky_engine,aam/engine,devoncarew/engine,chinmaygarde/flutter_engine,iansf/sky_engine,qiankunshe/sky_engine,takaaptech/sky_engine,Hixie/sky_engine,abarth/sky_engine,zhangxq5012/sky_engine,flutter/engine,cdotstout/sky_engine,mikejurka/engine,krisgiesing/sky_engine,devoncarew/sky_engine,jamesr/flutter_engine,tvolkert/engine,jimsimon/sky_engine,rmacnak-google/engine,jimsimon/sky_engine,mxia/engine,zhangxq5012/sky_engine,takaaptech/sky_engine,flutter/engine,mikejurka/engine,qiankunshe/sky_engine,abarth/sky_engine,mikejurka/engine,jason-simmons/flutter_engine,jamesr/sky_engine,mdakin/engine,cdotstout/sky_engine,jamesr/sky_engine,mdakin/engine,xunmengfeng/engine,devoncarew/sky_engine,mpcomplete/flutter_engine,chinmaygarde/sky_engine,krisgiesing/sky_engine,TribeMedia/sky_engine,flutter/engine,jason-simmons/sky_engine,iansf/sky_engine,axinging/sky_engine,mxia/engine,devoncarew/engine,rmacnak-google/engine,takaaptech/sky_engine,mikejurka/engine,xunmengfeng/engine,mpcomplete/engine,flutter/engine,devoncarew/sky_engine,mxia/engine,tvolkert/engine,mpcomplete/engine,zhangxq5012/sky_engine,mikejurka/engine,TribeMedia/sky_engine,krisgiesing/sky_engine,jamesr/sky_engine,Hixie/sky_engine,axinging/sky_engine,rmacnak-google/engine,TribeMedia/sky_engine,rmacnak-google/engine,jason-simmons/flutter_engine,zhangxq5012/sky_engine,jason-simmons/sky_engine,abarth/sky_engine,jason-simmons/sky_engine,jamesr/flutter_engine,mxia/engine,Hixie/sky_engine,rmacnak-google/engine,krisgiesing/sky_engine,mikejurka/engine,zhangxq5012/sky_engine,mpcomplete/engine,lyceel/engine,takaaptech/sky_engine,jimsimon/sky_engine,abarth/sky_engine,xunmengfeng/engine,mpcomplete/engine,qiankunshe/sky_engine,afandria/sky_engine,xunmengfeng/engine,jamesr/flutter_engine,jimsimon/sky_engine,TribeMedia/sky_engine,afandria/sky_engine,devoncarew/sky_engine,mpcomplete/flutter_engine,jason-simmons/sky_engine,chinmaygarde/sky_engine,iansf/sky_engine,Hixie/sky_engine,Hixie/sky_engine,cdotstout/sky_engine,axinging/sky_engine,tvolkert/engine,jamesr/flutter_engine,lyceel/engine,jimsimon/sky_engine,jamesr/flutter_engine,iansf/sky_engine,aam/engine,mpcomplete/engine,rmacnak-google/engine,mdakin/engine,qiankunshe/sky_engine,aam/engine,jason-simmons/sky_engine,jamesr/sky_engine,axinging/sky_engine,TribeMedia/sky_engine,mxia/engine,chinmaygarde/sky_engine,mpcomplete/engine,chinmaygarde/flutter_engine,cdotstout/sky_engine,lyceel/engine,mxia/engine,takaaptech/sky_engine,Hixie/sky_engine,tvolkert/engine,zhangxq5012/sky_engine,iansf/sky_engine,iansf/sky_engine,axinging/sky_engine,jamesr/flutter_engine,qiankunshe/sky_engine,jamesr/flutter_engine,jason-simmons/flutter_engine,TribeMedia/sky_engine,takaaptech/sky_engine,TribeMedia/sky_engine,devoncarew/sky_engine,devoncarew/sky_engine,krisgiesing/sky_engine,iansf/sky_engine,mpcomplete/flutter_engine,mdakin/engine,chinmaygarde/flutter_engine,TribeMedia/sky_engine,jamesr/flutter_engine,jimsimon/sky_engine,rmacnak-google/engine,tvolkert/engine,mdakin/engine,jamesr/sky_engine,axinging/sky_engine,abarth/sky_engine,axinging/sky_engine,jimsimon/sky_engine,abarth/sky_engine,mpcomplete/engine,qiankunshe/sky_engine,devoncarew/engine,aam/engine,mdakin/engine,TribeMedia/sky_engine,mikejurka/engine,afandria/sky_engine,mpcomplete/flutter_engine,chinmaygarde/sky_engine,lyceel/engine,jason-simmons/flutter_engine,Hixie/sky_engine,afandria/sky_engine,lyceel/engine,mikejurka/engine,axinging/sky_engine,mdakin/engine,mxia/engine,jason-simmons/flutter_engine,lyceel/engine,chinmaygarde/sky_engine,chinmaygarde/flutter_engine,cdotstout/sky_engine,flutter/engine,qiankunshe/sky_engine,flutter/engine,afandria/sky_engine,aam/engine,zhangxq5012/sky_engine,jamesr/sky_engine,Hixie/sky_engine,chinmaygarde/flutter_engine,takaaptech/sky_engine,jason-simmons/flutter_engine,jamesr/flutter_engine,cdotstout/sky_engine