Version 3.27.28 (based on bleeding_edge revision r21820)

Add v8::Promise::Then (Chromium issue 371288).

Performance and stability improvements on all platforms.

git-svn-id: https://v8.googlecode.com/svn/trunk@21823 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Merge remote-tracking branch 'upstream/trunk' into trunk