GitLab

Created a new class AppWebContentsHelper and moved the implementation of
several WebContentsDelegate methods (those which are not specific to app
windows) from AppWindow to there. AppWindow just calls those methods.
This will enable custom launcher pages to implement WebContentsDelegate
without duplicating a lot of code from AppWindow.

BUG=399116

Review URL: https://codereview.chromium.org/423453002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287302 0039d316-1c4b-4281-b951-d872f2087c98