Introduce a external allocation limit.

Introduce a external allocation limit.

If V8 is holding on to a lot of external memory, we attempt to clean
it up even if we do not get an allocation failure.  Since tiny V8
objects can hold on to a lot of external memory, we might run out of
external memory while waiting for a normal allocation failure.
Review URL: http://codereview.chromium.org/155916

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2519 ce2b1a6d-e550-0410-aec6-3dcde31c8c00