Remove WTF::String ConstructFromLiteralTag

Remove WTF::String ConstructFromLiteralTag (and the corresponding constructor) to simplify WTF::String class and as it does not have callers.

Review URL: https://chromiumcodereview.appspot.com/23647012

git-svn-id: svn://svn.chromium.org/blink/trunk@157522 bbb929c8-8fbe-4397-9dbb-9b2b20218538