Add an optional set_hook argument to defineProperty.

Allows classes to process a property set before the propertyChanged event is dispatched.

BUG=
TEST=

Review URL: http://codereview.chromium.org/6246078

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@74858 0039d316-1c4b-4281-b951-d872f2087c98