Canvas: Fix a regression with fillText + emoji

TextRun::NormalizedUTF16() unexpectedly replaced ZWJ and VS16 with ZWS.

This CL adds TreatAsZeroWidthSpaceInComplexScriptLegacy(), which is
identical to TreatAsZeroWidthSpaceInComplexScript() before the
regression, and uses it in NormalizedUTF16().

Bug: 331680200, 340641253
Change-Id: Ia5b83f4b14d3336c7cf62d8de2e5ca5266dcb978
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5546859
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1302447}