Remove String::Visit

R=yangguo@chromium.org

BUG=

Review URL: https://codereview.chromium.org/254763008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21046 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Return MaybeHandle from Object::ToSmi.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/260083011

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21047 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Merge remote-tracking branch 'upstream/master'