StringCache is now safe wrt weak persistents.  This CL updates it to reflect that as well as making it impossible to leak StringImpls.


R=jochen@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/20921004

git-svn-id: svn://svn.chromium.org/blink/trunk@155366 bbb929c8-8fbe-4397-9dbb-9b2b20218538