Extend ScalarValueString handling to include constructors.

ScalarValueString conversion may raise an exception; take that into
account when generating the constructor's code.

Also, common up various string-like and exception-raising tests
internally, making them properties over the underlying IDL types.

R=jsbell@chromium.org,haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/338893004

git-svn-id: svn://svn.chromium.org/blink/trunk@176732 bbb929c8-8fbe-4397-9dbb-9b2b20218538