Extracted this out of #5649
This commit was created with the following commands:
```
find . -iname "*.webidl" -type f -print0 | xargs -0 sed -i '' 's/http:\(.*\)whatwg.org/https:\1whatwg.org/g'
```
```
find . -iname "*.rs" -type f -print0 | xargs -0 sed -i '' 's/http:\(.*\)whatwg.org/https:\1whatwg.org/g'
```
For the majority of these cases, `as_slice` can be removed due to
`Deref`. In particular, `Deref` for:
* `String` -> `str`
* `Atom` -> `str`
The latter of those two requires, a bump of the locked `string-cache`
library