Allow tests within WebKit to implicitly convert WebCString to std::string.

Non-WebKit users can already do this.

BUG=172011

Review URL: https://chromiumcodereview.appspot.com/19047002

git-svn-id: svn://svn.chromium.org/blink/trunk@154155 bbb929c8-8fbe-4397-9dbb-9b2b20218538