The Object.getOwnPropertySymbols() calls into C++ at least once on every
possible path, so no point in having the JavaScript wrapper.

Drive-by-cleanup: Also move Symbol.prototype creation to C++ as well.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1587153003

Cr-Commit-Position: refs/heads/master@{#33318}