12806 Move initialization into FlutterEngine (cla: yes)

12808 Added an embedder example (cla: yes)

12809 Use the x64 host toolchain for x86 target gen_snapshot only on Linux (cla: yes)

12811 [web] Implement basic radial gradient (TileMode.clamp, no transform) (cla: yes)

12813 Unblock SIGPROF on flutter_tester start (cla: yes)

12814 Enable all engine test on windows (cla: yes)

12815 Revert “Adding Link SemanticsFlag” (cla: yes)

12816 Enable sanitizer build variants. (cla: yes)

12819 Open source canvas tests from flutter_web_ui (cla: yes)