Clean up redundant header files inclusion in navigatorcontentutils

In previous CLs, redundant header files were included by files of navigator content utils.
This cl removes those files as well as redundant forward declaration. Additionally this cl uses forward declaration for KURL in
NavigatorContentUtilsClientMock.h. Finally this cl adds OwnPtr.h to NavigatorContentUtils.h because blink coding style prefers to include directly when oilpan is disabled.


BUG=none

Review URL: https://codereview.chromium.org/365223002

git-svn-id: svn://svn.chromium.org/blink/trunk@177510 bbb929c8-8fbe-4397-9dbb-9b2b20218538