Improve the error message for unbounded viewports (#8321)
Improve the error message for unbounded viewports (#8321)

Rather than generating a cryptic assert, we now generate a more useful error
message.

Fixes #8296
flutter/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,cbracken/flutter,turnidge/flutter,Hixie/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,tvolkert/flutter,cbracken/flutter,flutter/flutter,tvolkert/flutter,turnidge/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,cbracken/flutter,flutter/flutter,flutter/flutter,turnidge/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,turnidge/flutter,Hixie/flutter,tvolkert/flutter,flutter/flutter,cbracken/flutter,cbracken/flutter,cbracken/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,turnidge/flutter,Hixie/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter