Introduce mock test system for navigator content utils

This cl introduces setNavigatorContentUtilsClientMock() and NavigatorContentUtilsClientMock class in order to simulate registerProtocolHandler(), unregisterProtocolHandler(), and isProtocolHandlerRegistered(). Thus, navigator content utils can test if defined functions can work correctly.

Besides this cl updates all navigator content util's tests so that the functions can work correctly.

BUG=none

Review URL: https://codereview.chromium.org/293403002

git-svn-id: svn://svn.chromium.org/blink/trunk@175454 bbb929c8-8fbe-4397-9dbb-9b2b20218538