19899 Fix invalid selection handling in text plugins (cla: yes)

19913 [CanvasKit] Mark used SkPictures so they aren’t deleted on the next frame. (cla: yes)

19915 Update 1.20 branch engine hash to Dart 2.9.0-21.10.beta (cla: yes)

19919 Ensure threads are merged when tearing down the Rasterizer (cla: yes, platform-ios, waiting for tree to go green)

19924 [web] Enable canvas measurement by default (cla: yes, platform-web)

19927 [web] Fixes arc scaling when counter clockwise (cla: yes)

19948 [web] Prevent crash when restore is called incorrectly after recording ends. (cla: yes)