When the 'y' flag was shipped, the property RegExp.prototype.unicode was
accidentally also shipped. However, the existence of this property should
be a usable feature testing point. This patch adds the 'unicode' getter on
RegExp.prototype only if the --harmony-regexp-unicode flag is turned on.

R=cbruni
CC=yangguo
BUG=v8:4644
LOG=Y

Review URL: https://codereview.chromium.org/1550713002

Cr-Commit-Position: refs/heads/master@{#33049}