Merged r14027 into 3.16 branch.

Always mark the entire valid prefix of the descriptor array.

BUG=chromium:196331

R=jkummerow@chromium.org

Review URL: https://chromiumcodereview.appspot.com/12995020

git-svn-id: https://v8.googlecode.com/svn/branches/3.16@14046 ce2b1a6d-e550-0410-aec6-3dcde31c8c00