Remove unmarked entries from per context map caches.

Remove unmarked entries from per context map caches.

Made "map_cache" a weak field of global context and added a pass over
all caches late in the marking phase.

R=vegorov@chromium.org
BUG=v8:1516
TEST=cctest/test-api/Regress1516

Committed: http://code.google.com/p/v8/source/detail?r=8515

[Vyacheslav Egorov (Chromium), 2011-07-01 12:18:20]

LGTM

We are cleaning the caches before processing weak references so we might end up
cleaning more than actually will survive the collection. But this should not
affect correctness.

[Vitaly Repeshko, 2011-07-01 12:33:12]

On 2011/07/01 12:18:20, Vyacheslav Egorov wrote:
> LGTM
> 
> We are cleaning the caches before processing weak references so we might end
up
> cleaning more than actually will survive the collection. But this should not
> affect correctness.

Thanks! As we discussed this weak reference processing doesn't mark any objects.
Still it's a bit sad we have two traversals over the context list now. But I
don't see a good way to simplify it.


-- Vitaly