Remove deprecated Clear/Dispose calls from ScopedPersistent.h

BUG=324225
R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/94943002

git-svn-id: svn://svn.chromium.org/blink/trunk@162836 bbb929c8-8fbe-4397-9dbb-9b2b20218538