Fix two nits with mojo::shell::app_container

app_container.cc should include its primary header first (to ensure the header
includes all of its dependencies) and the WeakPtrFactory should be the last
member in the class (because weak_ptr.h says so).

R=thakis@chromium.org

Review URL: https://codereview.chromium.org/29843005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229625 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229625