21164 Add an explicit API for font change notification (cla: yes)

21166 Define _USE_MATH_DEFINES on Windows where needed (cla: yes, waiting for tree to go green)

21167 Force builders to run. (cla: yes)

21169 Roll Fuchsia Mac SDK from arL8NjPHW… to 8q8xxvY60… (cla: yes, waiting for tree to go green)

21170 Roll Skia from 37fd658981dd to 2bc4077c9e42 (4 revisions) (cla: yes, waiting for tree to go green)

21171 Roll Fuchsia Linux SDK from zKQw_oHsx… to -iBBiPj1C… (cla: yes, waiting for tree to go green)