See http://tc39.github.io/ecma262/#prod-RegExpUnicodeEscapeSequence

R=erik.corry@gmail.com, erikcorry@chromium.org
BUG=v8:2952
LOG=N

Review URL: https://codereview.chromium.org/1681893002

Cr-Commit-Position: refs/heads/master@{#33892}