This is done by reimplementing Symbol.for directly on top of
NameDictionary. A nice side effect is the removal of src/js/symbol.js

R=jochen@chromium.org, peria@chromium.org
BUG=chromium:617892

Review-Url: https://codereview.chromium.org/2551763003
Cr-Commit-Position: refs/heads/master@{#41520}