IDL: [Unforgeable] on operation should imply DontDelete+ReadOnly

Previously, the only effect of [Unforgeable] on an operation was to put
the property on instance objects instead of on the prototype object.

All actual unforgeable operations -- Location's assign(), replace(),
reload(), toString() and valueOf() -- were additionally declared read-
only using [ReadOnly], which as a side-effect also made the properties
unconfigurable ("dont-delete").

This CL fixes [Unforgeable] on operations, and drops the then redundant
[ReadOnly] attributes from Location.idl. No CG changes.

BUG=

Review URL: https://codereview.chromium.org/350683004

git-svn-id: svn://svn.chromium.org/blink/trunk@176940 bbb929c8-8fbe-4397-9dbb-9b2b20218538