This is the main patch for the bug. It aims to change the grapheme cluster
break's `Next()` API by implementing SegmentIteratorUtf16 interface, and adapt
the callers. It shouldn't change the behavior.
While rewriting the caller, one caveat worth mentioning is the loop termination
condition. If the old code relies on `!AtEnd()` as the loop termination
condition, and it advances the iterator at the end of the loop, it meant
to *skip* its logic when the break position is at the end of the string. For
example, see the `mozTXTToHTMLConv::NumberOfMatches`.
This patch also hooks grapheme cluster break iterator into
Segmenter::TryCreate() interface.
Existing test coverage for the file changed:
- netwerk/test/unit/test_mozTXTToHTMLConv.js
- layout/reftests/forms/input/file/dynamic-max-width.html
Differential Revision: https://phabricator.services.mozilla.com/D135643
653f4b3694