Enable ES6 Map and Set by default

In doing so also remove all references to the --harmony-collections flag.
Due to the way context snapshotting works, it's not possible to simply
enable the flag by default.

Depends on ES6 Symbols: https://codereview.chromium.org/421313004

BUG=v8:1622
LOG=Y
R=arv@chromium.org, rossberg@chromium.org

Review URL: https://codereview.chromium.org/427723002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22889 ce2b1a6d-e550-0410-aec6-3dcde31c8c00