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This change modifies zxcvbn's repeat_match to correctly detect
repeated sequences of non-ASCII characters. Since std::regex does not
natively understand UTF8 codepoints, and re2 does not support the
required backreferences, this change updates the regex engine to use
icu::Regex instead.

Furthermore, since zxcvbn only intends to supports UTF8 strings, the
fuzzers are modified to only produce valid UTF8.

TBR=dpranke

Fixed: 1111771
Change-Id: I4a9985997b6d6cd33558cf8607e195e61dac8109
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377934
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Alex Gough <ajgo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806351}