Switch icu repo to icu.git in v8 DEPS.

This ports https://codereview.chromium.org/769413004 to v8
DEPS.

BUG=chromium:438401
LOG=n
TBR=jshin@chromium.org

Review URL: https://codereview.chromium.org/796813002

Cr-Commit-Position: refs/heads/master@{#25766}