Every place where AllocationMemento is initialized with an AllocationSite is now checked to be sure a valid Site goes in. This is temporary code to diagnose chromium bug 284577.

Performance and stability improvements on all platforms.

git-svn-id: http://v8.googlecode.com/svn/trunk@16723 ce2b1a6d-e550-0410-aec6-3dcde31c8c00