Fix a TextIterator crash in Greek text-transform

CSS text-transform can change text length in either ways, resulting in
more or fewer characters in LayoutText. The former case is more well
known and has been largely worked around.

Now that we see the latter case being possible, this patch works around
it in TextIterator to avoid reaching a NOTREACHED().

Fixed: 1424324
Bug: 750990
Change-Id: I4b8ba886bcacc06e9e491badb30af98d0f22c37c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4351287
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Auto-Submit: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1119018}