This was only used by Heap::CollectGarbage to check if after GC
available space is bigger than was requested, but nobody checked
the value returned by Heap::CollectGarbage, so requested size
was efficiently unused.

However, it may trigger spurious out of memory exceptions if requested
size is big enough.

BUG=http://code.google.com/p/chromium/issues/detail?id=54580

Review URL: http://codereview.chromium.org/3836001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5642 ce2b1a6d-e550-0410-aec6-3dcde31c8c00