Version 3.28.12 (based on bleeding_edge revision r22210)

Use the HeapObjectIterator to scan-on-scavenge map pages (Chromium issue 390732).

Introduce debug events for Microtask queue (Chromium issue 272416).

Performance and stability improvements on all platforms.

git-svn-id: https://v8.googlecode.com/svn/trunk@22212 ce2b1a6d-e550-0410-aec6-3dcde31c8c00