29876 Reset viewport_metrics when spawning new engine (cla: yes)

29878 Roll Dart SDK from b1afd0fae784 to 307a0e735317 (1 revision) (cla: yes, waiting for tree to go green)

29881 Roll Skia from 7ecacbc4c6be to e00afb0a1a68 (14 revisions) (cla: yes, waiting for tree to go green)

29883 Roll Dart SDK from 307a0e735317 to 314057a2d7d3 (2 revisions) (cla: yes, waiting for tree to go green)