This CL addresses the github issue [#8946](https://github.com/google/ExoPlayer/issues/8964). That issue requests support for `font-size` CSS property in WebVTT subtitle format. This CL:
* Adds support for `font-size` property by extending capabilities of WebVTT `CssParser`. Implementation of `font-size` property value parsing is based on the one in `TtmlDecoder`.
* Adds unit test along with test file containing WebVTT subtitles with all currently supported `font-size` units.
#minor-release
PiperOrigin-RevId: 388423859
