Add `Harness::new_ui`, `Harness::fit_contents` (#5301) #819

Job	Run time
android	22s
Check wasm32 + wasm-bindgen	2m 28s
cargo-deny aarch64-apple-darwin	40s
Check wasm32+atomics	1m 52s
cargo-deny aarch64-linux-android	42s
Format + check + test	31s
ios	21s
cargo-deny i686-pc-windows-gnu	41s
Run tests	40s
cargo-deny i686-pc-windows-msvc	40s
Check Windows	1m 16s
cargo-deny i686-unknown-linux-gnu	41s
cargo-deny wasm32-unknown-unknown	40s
cargo-deny x86_64-apple-darwin	38s
cargo-deny x86_64-pc-windows-gnu	37s
cargo-deny x86_64-pc-windows-msvc	36s
cargo-deny x86_64-unknown-linux-gnu	37s
cargo-deny x86_64-unknown-linux-musl	38s
cargo-deny x86_64-unknown-redox	42s
	15m 22s

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