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This CL replaces third_party/icu uses with base/third_party/icu.
Along with the change, base/third_party/icu/* is updated to v67.
- Extracted several macros used for char_iterator implementation.
- Removed utf8_nextCharSafeBody, as it is no longer used.

This CL is preparation to move char_iterator to base/strings
so that it can be used in new functions in base/.

Bug: 2492481
Test: Ran base_unittests.
Change-Id: I779c7384b15701dcac3feac9d258be5793ca209d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2494402
Commit-Queue: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821661}