Fixed usage of GridLayout for SimpleWebViewDialog.

BUG=294477
TEST=interactive_ui_tests:CaptivePortalWindow*

Review URL: https://codereview.chromium.org/143963004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246056 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246056