Weak Cells

Introduce an object that holds a weak reference.
Design document: http://goo.gl/9dSvvy.

BUG=
R=erik.corry@gmail.com

Review URL: https://codereview.chromium.org/640303006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24606 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Fix V8's default timer event logger.

This broke because the optimizing compiler thread no longer holds
Isolate::Current() in its TLS.

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/655813002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24607 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Catch exceptions thrown when enqueuing change records.

R=ishell@chromium.org
BUG=chromium:417709
LOG=N

Review URL: https://codereview.chromium.org/653593002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24608 ce2b1a6d-e550-0410-aec6-3dcde31c8c00