Avoid duplication of a hidden & inherited prototype's properties.

In Runtime_GetLocalPropertyNames(), the hidden prototypes of an object
are also consulted when deriving the property name set. However, if
given a function object and its template was inherited from the
template of one of its hidden prototypes, that hidden prototype's
property accessors will be present on the object already. Unwanted
duplicates will therefore appear.

Hence, go through the property names that the hidden prototypes
contribute and remove any already occurring ones.

Assumed to be a rare constellation, so the cost of this extra pass is
considered acceptable.

LOG=N
R=dcarney@chromium.org, jochen@chromium.org, rossberg@chromium.org
BUG=269562

Review URL: https://codereview.chromium.org/116533003

Patch from Sigbjorn Finne <sof@opera.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18448 ce2b1a6d-e550-0410-aec6-3dcde31c8c00