app_shell: Add AppShellContentClient.

Add a simple AppShellContentClient class and update
AppShellMainDelegate to use it instead of
content::ShellContentClient.

BUG=305404
TBR=darin@chromium.org

Review URL: https://codereview.chromium.org/26690003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228133 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228133