[LayoutNG] Fix NextOnLineInternalNG

Both this and PreviousOnLineInlineNG had the same typo, but
PreviousOnLineInlineNG was fixed in r659579
(http://crrev.com/c/1608867).

Unlike http://crrev.com/c/1608867, unfortunately this does
not seem to have a test coverage.

Change-Id: I707884c83e9c5bf711def37ceee888a294409e41
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1739235
Commit-Queue: Koji Ishii <kojii@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685207}