Remove unnecessary ContentBrowserClient in test.

DevToolsManagerTest was forcing a process swap using
ShouldSwapProcessesForNavigation, but this was unnecessary since the
navigation was cross-site anyway.  Removing the extra code so that it doesn't
force an unnecessary swap on the first navigation.

BUG=123007
TEST=DevToolsManagerTest.ReattachOnCancelPendingNavigation

Review URL: https://codereview.chromium.org/53293004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232916 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232916