Some tests involving AllocationSites are failing in GcStress test mode.
The reason is that an AllocationMemento associated with an array only
lives for one gc (it is unrooted). So an excess of garbage collections
in these tests cause the Memento to be lost, and expected behavior
can't be guaranteed.

BUG=
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/19544002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15714 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
The gc should be able to traverse all AllocationSites for decision making. The sites are threaded into a weak list. Special problems include:

* Allocations of AllocationSites occur in generated code, so generated code needs to be able to add to the list. For now I have a special hydrogen instruction, though it would be nice to use general purpose instructions.
* The snapshot contains AllocationSites, and these need to be re-threaded into the list on deserialization.

Something nice is that the AllocationSites are only created in old space, so a special new space visitor isn't required.

BUG=
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/18173013

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15715 ce2b1a6d-e550-0410-aec6-3dcde31c8c00