Fix property enum cache creation to include only own properties

With this fix, we only create the enum cache for own property descriptors (originally we cached all descriptors in the map).  The problem was that the size of all descriptors could be trimmed during GC triggered by allocating the storage for the cache, so we could have ended up with a wrong storage size.

This is really Toon's fix, I have only created a small repro case.

BUG=
R=verwaest@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=20308

[Toon Verwaest, 2014-03-27 11:53:38]

lgtm with 2 nits

https://codereview.chromium.org/212673011/diff/1/src/handles.cc
File src/handles.cc (right):

https://codereview.chromium.org/212673011/diff/1/src/handles.cc#newcode666
src/handles.cc:666: int num_enum =
map->NumberOfDescribedProperties(OWN_DESCRIPTORS, DONT_SHOW);
This valid is already available as own_property_count right above.

https://codereview.chromium.org/212673011/diff/1/src/handles.cc#newcode707
src/handles.cc:707: object->map()->SetEnumLength(index);
use own_property_count here.

[Jarin, 2014-03-27 15:33:14]

Committed patchset #2 manually as r20308 (presubmit successful).