Roll ICU to 7f873c45

There's only one change in the roll.

 https://chromium.googlesource.com/chromium/deps/icu/+log/08cb9568..7f873c45

TBR=mark@chromium.org

Bug: 770452
Test: See the bug
Change-Id: I6e9c1b8a7e0d2f013517402e7cd7dd434f398b27
Reviewed-on: https://chromium-review.googlesource.com/709939
Reviewed-by: Jungshik Shin <jshin@chromium.org>
Commit-Queue: Jungshik Shin <jshin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#507752}