Use a consistent scheme name in navigatorcontentutils tests

Tests regarding the navigatorcontentutils have used two names for scheme, "protocol" and "scheme".
This may cause confusion for developers. So, this cl changes "protocol" name with "scheme" in the tests.

BUG=none

Review URL: https://codereview.chromium.org/440833006

git-svn-id: svn://svn.chromium.org/blink/trunk@179579 bbb929c8-8fbe-4397-9dbb-9b2b20218538