Added browser test for Captive Portal window ctor and dtor.

BUG=294477
TEST=browser_tests:CaptivePortalWindowCtorDtor*

Review URL: https://codereview.chromium.org/163413002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251071 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251071