Add defensive assert for having a weak, empty presistent in a

Add defensive assert against a weak callback on an empty persistent.

(This shouldn't happen in the first place, as by definition that callback would never be called. However, the referenced bug sorta looks like this happened. If so, the CHECK should help us pinpoint the culprit.)

R=jochen@chromium.org
BUG=368095
LOG=N

Committed: https://code.google.com/p/v8/source/detail?r=21521

[jochen (gone - plz use gerrit), 2014-05-05 17:42:46]

https://codereview.chromium.org/265823006/diff/1/include/v8-util.h
File include/v8-util.h (right):

https://codereview.chromium.org/265823006/diff/1/include/v8-util.h#newcode288
include/v8-util.h:288: RELEASE_ASSERT(!persistent->IsEmpty());
the v8 equivalent to RELEASE_ASSERT() is CHECK(), however, it's not available in
the header :-/

I'd add the CHECK in GlobalHandles::MakeWeak()

[vogelheim, 2014-05-06 11:55:07]

PTAL.

That first version wasn't meant for review, as both you and the compiler
noticed.. :-P

This triggers a minor bug in the webkit tests, so I'll have to wait for that to
be fixed before submitting.

[jochen (gone - plz use gerrit), 2014-05-06 11:58:02]

lgtm

[vogelheim, 2014-05-27 12:26:52]

Committed patchset #2 manually as r21521 (presubmit successful).