Pull out LazyLineBreakIterator's prior context handling into a separate class
<rdar://problem/109646131>
Pull request: https://github.com/WebKit/WebKit/pull/14169
Committed 264444@main (b5583c5d124d): <https://commits.webkit.org/264444@main> Reviewed commits have been landed. Closing PR #14169 and removing active labels.