Make AdjustAmountOfExternalAllocatedMemory() more robust.
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