Previously, the hash was stored on the underlying global object, since
it was stored in the hidden property table. This patch moves to an
implementation modeled on JSProxy, adding a new 'hash' field to JSGlobalProxy.

This allows storing the global proxy in a Map, Set, WeakMap, or WeakSet and
accessing it even after the proxy has been attached to a new global, which
is Firefox's current behavior and was the consensus of a recent thread on public-script-coord:
http://lists.w3.org/Archives/Public/public-script-coord/2014AprJun/0012.html

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/254433002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@21150 ce2b1a6d-e550-0410-aec6-3dcde31c8c00