Remove calls to the deprecated 'v8::String::NewSymbol' from Blink.

BUG=324225
R=marja@chromium.org

Review URL: https://codereview.chromium.org/96613002

git-svn-id: svn://svn.chromium.org/blink/trunk@162885 bbb929c8-8fbe-4397-9dbb-9b2b20218538