Notification of metrics changes.
Notification of metrics changes.

The Rendering layer binding now multiplexes the dart:ui metrics change
notification so that multiple consumers can all listen to the change.

flutter/flutter,flutter/flutter,jason-simmons/flutter,cbracken/flutter,aghassemi/flutter,turnidge/flutter,Hixie/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,Hixie/flutter,Hixie/flutter,turnidge/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,flutter/flutter,flutter/flutter,tvolkert/flutter,tvolkert/flutter,jason-simmons/flutter,aghassemi/flutter,tvolkert/flutter,Hixie/flutter,jason-simmons/flutter,jason-simmons/flutter,Hixie/flutter,flutter/flutter,turnidge/flutter,turnidge/flutter,cbracken/flutter,flutter/flutter,flutter/flutter,aghassemi/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,turnidge/flutter,aghassemi/flutter,Hixie/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter