25853 fuchsia: Fix bad syntax in viewStateConnected msg (cla: yes, platform-fuchsia)

25860 Moved PlatformMessage’s to unique_ptrs (platform-ios, platform-android, cla: yes, waiting for tree to go green, platform-fuchsia, embedder)

25867 Switch PlatformMessages to hold data in Mappings (platform-ios, platform-android, cla: yes, platform-macos, platform-fuchsia, embedder)

25868 A11y inspect (cla: yes, platform-fuchsia)

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

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