DevTools: Display packaged app's background page in chrome://inspect#app

BUG=315785

Review URL: https://codereview.chromium.org/64263002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@233628 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 233628