WebString::utf8() should return a std::string

There don't appear to be any strong reasons for prefering WebCString over
std::string in the API. To improve integration with Chromium, we're going to
move the API over to using std::string over time.

This first callsite is useful to migrate because it makes it possible to
optimize the conversion of WebURL into GURL.

R=darin
BUG=261412

Review URL: https://chromiumcodereview.appspot.com/20176003

git-svn-id: svn://svn.chromium.org/blink/trunk@154886 bbb929c8-8fbe-4397-9dbb-9b2b20218538