Create a skeleton AppBannerManager class

Create a skeleton AppBannerManager class

* Adds an AppBannerManager that will be used to manage a single
  AppBannerView.
* Each AppBannerManager is owned by a particular TabBase and is
  alerted to changes its TabBase by a TabObserver.
  The TabObserver's main use is to update the AppBannerManager
  when the WebContents has changed.

Note that this class is a skeleton: most of the real functionality
(like creating a AppBannerView) has been parted off.

BUG=341556

Review URL: https://codereview.chromium.org/155503004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251136 0039d316-1c4b-4281-b951-d872f2087c98