[tracing] SceneDisplayLag is a synchronous event (#17878)
[tracing] SceneDisplayLag is a synchronous event (#17878)

This change converts it from an events that spans a time interval
to an event that occurs at an instant.

We also emit this trace event when there is no lag as opposed to
only when there was a lag to make it monotonous.

Co-authored-by: Kaushik Iska <fbd80821f39aadb1dc40eb8dd97acd5d2f316cfd@google.com>
chinmaygarde/sky_engine,flutter/engine,flutter/engine,aam/engine,Hixie/sky_engine,chinmaygarde/sky_engine,tvolkert/engine,jamesr/sky_engine,tvolkert/engine,devoncarew/sky_engine,rmacnak-google/engine,Hixie/sky_engine,aam/engine,Hixie/sky_engine,chinmaygarde/sky_engine,jason-simmons/sky_engine,flutter/engine,rmacnak-google/engine,devoncarew/sky_engine,chinmaygarde/sky_engine,tvolkert/engine,devoncarew/engine,chinmaygarde/sky_engine,tvolkert/engine,jamesr/sky_engine,chinmaygarde/sky_engine,jamesr/flutter_engine,chinmaygarde/flutter_engine,flutter/engine,chinmaygarde/flutter_engine,flutter/engine,jamesr/flutter_engine,aam/engine,rmacnak-google/engine,devoncarew/engine,jamesr/flutter_engine,aam/engine,jamesr/sky_engine,jason-simmons/sky_engine,jamesr/flutter_engine,jamesr/flutter_engine,devoncarew/engine,jason-simmons/flutter_engine,jamesr/sky_engine,devoncarew/engine,Hixie/sky_engine,jamesr/sky_engine,jamesr/sky_engine,tvolkert/engine,devoncarew/sky_engine,aam/engine,jamesr/flutter_engine,jason-simmons/flutter_engine,flutter/engine,jamesr/flutter_engine,Hixie/sky_engine,chinmaygarde/flutter_engine,jason-simmons/flutter_engine,jason-simmons/flutter_engine,Hixie/sky_engine,jason-simmons/sky_engine,rmacnak-google/engine,rmacnak-google/engine,devoncarew/engine,chinmaygarde/flutter_engine,devoncarew/sky_engine,chinmaygarde/flutter_engine,devoncarew/sky_engine,chinmaygarde/flutter_engine,chinmaygarde/flutter_engine,aam/engine,rmacnak-google/engine,jamesr/sky_engine,aam/engine,jason-simmons/flutter_engine,rmacnak-google/engine,jamesr/flutter_engine,jason-simmons/sky_engine,chinmaygarde/sky_engine,devoncarew/engine,jason-simmons/flutter_engine,flutter/engine,jason-simmons/sky_engine,devoncarew/sky_engine,tvolkert/engine,Hixie/sky_engine,Hixie/sky_engine,tvolkert/engine,devoncarew/sky_engine,jason-simmons/flutter_engine,flutter/engine,jason-simmons/flutter_engine,jason-simmons/sky_engine,aam/engine,devoncarew/engine,jason-simmons/sky_engine,jamesr/flutter_engine