Remove moar deprecated uses of 'v8::String::NewSymbol' from Blink.

BUG=324225
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/96693003

git-svn-id: svn://svn.chromium.org/blink/trunk@162900 bbb929c8-8fbe-4397-9dbb-9b2b20218538