11392 Wire up software rendering in the test compositor. (cla: yes)

11394 Avoid root surface acquisition during custom composition with software renderer. (cla: yes)

11395 Remove deprecated ThreadTest::GetThreadTaskRunner and use the newer CreateNewThread API. (cla: yes)

11413 Ios simulator unittests seem to not consider the full compilation unit (cla: yes)

11416 Shrink cirrus docker image: reduce RUN count, apt-get clean (cla: yes)

11419 Support non-60 refresh rate on PerformanceOverlay (cla: yes)

11420 Fix touchpad scrolling on Chromebook  (cla: yes)

11421 sync Flutter Web engine to the latest (cla: yes)