Rolling v8/third_party/icu: https://chromium.googlesource.com/chromium/deps/icu/+log/31c77cb..bbdc7d8

Add microsecond and nanosecond unit (Frank Tang)
https://chromium.googlesource.com/chromium/deps/icu/+/bbdc7d8

R=v8-waterfall-sheriff@grotations.appspotmail.com,mtv-sf-v8-sheriff@grotations.appspotmail.com,ftang@chromium.org

Change-Id: I2d996e16a0e3e52c1de237eb13ca656829ff3d0c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3857874
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