21798 [ios] Create a standalone external view embedder on iOS (cla: yes, platform-ios, waiting for tree to go green)

21801 Roll Skia from f58db3c94da3 to 387fd62a1280 (3 revisions) (cla: yes, waiting for tree to go green)

21802 Enable lazy-async-stacks by-default in all modes (Take 4) (cla: yes)

21803 Roll Skia from 387fd62a1280 to c89a7ee628db (1 revision) (cla: yes, waiting for tree to go green)

21804 Roll Skia from c89a7ee628db to fa8891164062 (1 revision) (cla: yes, waiting for tree to go green)