Improve SeqStringSetChar implementation.

R=jkummerow@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/15743006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14769 ce2b1a6d-e550-0410-aec6-3dcde31c8c00