Change a url parameter type with WebURL

As mentioned in https://codereview.chromium.org/18179009/#msg7, it would be good if we use WebURL for url parameters instead of using WebString. This cl change it with WebURL.

BUG=N/A

Review URL: https://codereview.chromium.org/133273030

git-svn-id: svn://svn.chromium.org/blink/trunk@165458 bbb929c8-8fbe-4397-9dbb-9b2b20218538