26623 Add support for native callbacks to the macOS embedder test harness (cla: yes, platform-macos)

26624 Roll Skia from 8aef107f6d58 to 2c9a6ec3a0d0 (1 revision) (cla: yes, waiting for tree to go green)

26625 Roll Fuchsia Linux SDK from qqVmyK6TU… to TqViQQzJo… (cla: yes, waiting for tree to go green)

26626 [web] Allow some more nulls in platform_dispatcher. (cla: yes, waiting for tree to go green, platform-web)

26627 Python 3 support for macOS/iOS builds (cla: yes, waiting for tree to go green)