Add `Message::is_maybe_writable` (#35340) #11161

Job	Run time
android (ubuntu-20.04, x86_64-linux-android)	2m 52s
ios (macos-11, aarch64-apple-ios)	6m 20s
android (ubuntu-20.04, aarch64-linux-android)	3m 11s
ios (macos-11, x86_64-apple-ios)	6m 19s
android (ubuntu-20.04, i686-linux-android)	2m 57s
ios (macos-11, aarch64-apple-darwin)	7m 8s
android (ubuntu-20.04, armv7-linux-androideabi)	3m 9s
ios (macos-11, x86_64-apple-darwin)	6m 18s
error_reporting	0s
	38m 14s

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