Merged r18284 into 3.23 branch.

Increase number of available major keys.

R=bmeurer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/136513002

git-svn-id: https://v8.googlecode.com/svn/branches/3.23@18561 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Merge remote-tracking branch 'upstream/3.23' into 3.23