Do not crash if called from a thread without V8 isolate, reset the external
memory counters in case of overflow, bump the external allocation limit.

This will allow us to track typed array allocation and deallocation in WebKit.

BUG=v8:2022,122097,42342
R=jkummerow@chromium.org

Review URL: https://chromiumcodereview.appspot.com/10837122

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12262 ce2b1a6d-e550-0410-aec6-3dcde31c8c00