Prevent changes to hidden properties from being observable via Object.observe

Prevent changes to hidden properties from being observable via Object.observe

This addresses the leak that mstarzinger points out (https://codereview.chromium.org/26390003/) and includes the test.

Note that this adds a test that observing changes to the empty-string property remains possible.

BUG=
R=mstarzinger@chromium.org, rossberg@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=17257

[rafaelw, 2013-10-16 22:34:52]

PTAL. Let me know if you'd like to see a helper for computing is_observed for
these cases which takes the object and key value.

[rossberg, 2013-10-17 09:10:05]

LGTM

Don't think a helper is necessary, considering that 2 thirds of the condition
should go away eventually. :)

[rossberg, 2013-10-17 09:10:06]

LGTM

Don't think a helper is necessary, considering that 2 thirds of the condition
should go away eventually. :)

[Michael Starzinger, 2013-10-17 11:35:03]

Drive-by LGTM.

[rafaelw, 2013-10-17 20:49:55]

Committed patchset #2 manually as r17257 (presubmit successful).