[line-clamp] Make `-webkit-line-clamp: none` parse

The valid values of the `-webkit-line-clamp` property are the keyword
`none` and a positive integer. Chromium represents it internally as an
`int`, where `none` maps to the zero value.

However, although the default value is set to zero, and zero
serializes to `none`, `none` is treated as an invalid value when
parsing. This means setting `-webkit-line-clamp: none` on a
declaration higher in the cascade order will not override other
declarations.

The reason for this seems to be that, if the `none` keyword was
parsed, converting it into an `int` would fail. However, for the
unprefixed `line-clamp` property, this very same issue was solved by
adding a `ConvertIntegerOrNone` converter in
https://crrev.com/c/5419122. This patch uses this converter on
`-webkit-line-clamp` as well.

Bug: 40336192
Change-Id: I05059830315821dbf0cab1bc3d3f253ad70b9c7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5844930
Commit-Queue: Andreu Botella <abotella@igalia.com>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1352760}