Add new Harmony methods to String.prototype object.

String.prototype.repeat
String.prototype.startsWith
String.prototype.endsWith
String.prototype.contains

http://people.mozilla.org/~jorendorff/es6-draft.html

BUG=v8:2796,v8:2797,v8:2798,v8:2799
TEST=mjsunit/string-repeat,mjsunit/string-startswith,mjsunit/string-endswith,mjsunit/string-contains
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/21014007

Patch from Ioseb Dzmanashvili <ioseb.dzmanashvili@gmail.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15960 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Mark maps as unstable if their instances potentially transition away.
Use this as a prerequisite for adding code dependencies.

R=ulan@chromium.org

Review URL: https://chromiumcodereview.appspot.com/21095005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15961 ce2b1a6d-e550-0410-aec6-3dcde31c8c00