27823 [web] Update CanvasPool documentation (cla: yes, platform-web, needs tests)

27825 Roll Dart SDK from a325ab04ed38 to fa724490f430 (1 revision) (cla: yes, waiting for tree to go green)

27827 Roll Skia from 126788e087af to ae2171eba699 (3 revisions) (cla: yes, waiting for tree to go green)

27828 Fix race condition in FlutterSurfaceManager (cla: yes, waiting for tree to go green, platform-macos, needs tests)

27830 Roll Skia from ae2171eba699 to 31df6806c06b (3 revisions) (cla: yes, waiting for tree to go green)