Ensure that we do a non-incremental GC before relying on weak handle callbacks.

Previously, some tests failed when being run with a low GC interval. This was
caused by a switch of the GC to incremental marking mode, which in turn did not
fire any callbacks for weak global handles. Now we make sure that we run in
non-incremental mode, although via a slightly misleading GC flag.

We should probably review the uses of PerformScavenge() and gc() in our tests to
see if they actually mean "make sure our callbacks fired".

Review URL: https://chromiumcodereview.appspot.com/9378007

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10681 ce2b1a6d-e550-0410-aec6-3dcde31c8c00