Add some sensible defaults to FixedColumnCountGrid and MaxTileWidthGrid
Add some sensible defaults to FixedColumnCountGrid and MaxTileWidthGrid

If we use null for these values, the implementations will later assert.



Fixes #3102
turnidge/flutter,aghassemi/flutter,Hixie/flutter,turnidge/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,turnidge/flutter,tvolkert/flutter,aghassemi/flutter,flutter/flutter,flutter/flutter,tvolkert/flutter,turnidge/flutter,Hixie/flutter,Hixie/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,Hixie/flutter,flutter/flutter,tvolkert/flutter,turnidge/flutter,cbracken/flutter,flutter/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,tvolkert/flutter,tvolkert/flutter,flutter/flutter,tvolkert/flutter,Hixie/flutter,Hixie/flutter,cbracken/flutter,aghassemi/flutter,aghassemi/flutter,tvolkert/flutter,cbracken/flutter,cbracken/flutter,Hixie/flutter,tvolkert/flutter,cbracken/flutter,Hixie/flutter,flutter/flutter,flutter/flutter,cbracken/flutter,Hixie/flutter,cbracken/flutter