25984 fuchsia: Fix multi-views fallout (cla: yes, platform-fuchsia)

25985 Add an allowlist flag for Skia trace event categories (cla: yes, waiting for tree to go green)

25987 Add image generator registry (cla: yes)

25988 Platform channels, eliminate objc copies (platform-ios, platform-android, cla: yes, waiting for tree to go green, platform-macos, platform-fuchsia, embedder)

25991 Make SceneBuilder.push* not return nullable objects (cla: yes, waiting for tree to go green, platform-web)

25993 Roll buildroot to pick change to cppwinrt invocation (cla: yes)

25994 [canvaskit] Set a maximum number of overlay canvases. (cla: yes, platform-web)