Refactor how `InlineNodeDataEditor` calls `mismatch`

This patch:
* Extracts the logic from `Run()` to `MatchedLengths()`.
* Moves `subpan` in `MismatchFromEnd` to `StringView` in
  `MatchedLengths()`.

This patch should have no behavior changes.

Bug: 325517313
Change-Id: Ic900655c639491244f524ec43832d93c5320bef6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5311581
Commit-Queue: Kent Tamura <tkent@chromium.org>
Auto-Submit: Koji Ishii <kojii@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1263826}