Refactor MediaConstraintsWebRTC::isConstraintValid.

 - Convert MediaConstraintsWebRTC::isConstraintValid to a static function.
 - Use ASCIILiteral instead of WTF::String.
 - Use a binary search instead of linear (ok, the array isn't too long :-) )
 - Keep the valid constraints array local.

Reviewed by: Sergio Correia
Turn member functions that don't need to be member functions into static functions.

Reviewed by: Daker Fernandes Pinheiro