Update V8 DEPS.

Rolling v8/third_party/icu to 51c1a4ce5f362676aa1f1cfdb5b7e52edabfa5aa

TBR=machenbach@chromium.org
BUG=

Review URL: https://codereview.chromium.org/816763003

Cr-Commit-Position: refs/heads/master@{#25894}