27189 MacOS: Release backbuffer surface when idle (cla: yes, waiting for tree to go green, platform-macos, needs tests)

27190 Roll Skia from ad77b4db8dc6 to 3037d9f322ff (1 revision) (cla: yes, waiting for tree to go green)

27191 MacOS (metal): Block raster thread instead of platform thread (cla: yes, waiting for tree to go green, platform-macos, needs tests)

27193 Roll Dart SDK from 4422591ed19b to 586ecc5a0697 (1 revision) (cla: yes, waiting for tree to go green)

27195 Roll Fuchsia Linux SDK from a1MOPse95… to CEpfImAgA… (cla: yes, waiting for tree to go green)