Following up on discussion at https://chromium-review.googlesource.com/c/chromium/src/+/986475

To repro:
- Enter ﾀﾞinto a textarea. This is U+FF80 "HALFWIDTH KATAKANA LETTER TA" and U+FF9E "HALFWIDTH KATAKANA VOICED SOUND MARK".
- Backspace

Expected: Entire grapheme is deleted
Actual: Only the sound mark is deleted

On Mac, this is inconsistent with native text editing.