Change --harmony-exponentiation-operator to be enabled by default.

Intent-to-Ship: https://groups.google.com/forum/#!topic/v8-users/W6rKEsdQHXE

BUG=v8:3915
LOG=N
R=adamk@chromium.org, rossberg@chromium.org

Review URL: https://codereview.chromium.org/1895113003

Cr-Commit-Position: refs/heads/master@{#35593}