Close the ui channel when desktop is activated.

Not doing this causes it to take a very long time on a Metro -> Desktop transition, due to the browser holding the channel.

BUG=328834

Review URL: https://codereview.chromium.org/108493004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242015 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242015
Convert remaining uses of WebView::initializeMainFrame to setMainFrame.

This is the Chrome-side change. Fixing this makes inverting the owning
relationship between WebFrame and Frame easier.

BUG=329266
R=ajwong@chromium.org, bauerb@chromium.org, boliu@chromium.org, ddorwin@chromium.org, scottbyer@chromium.org

Review URL: https://codereview.chromium.org/114773003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242016 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242016