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Add some comments about URLFixer, which should not be used on anything
other than user-typed strings.

Also warn that URLFixer deviates from GURL's logic with segmentation,
which leads to Chrome parsing strings differently from GURL and
Javascript's URL object. That is troublesome.

This CL makes no code changes, just commentary updates.

Bug: 1130091
Change-Id: Ib0302db96870c1fa4258541b2cd8a76a729b669a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2437071Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811721}