extensions: Create AppSorting interface.

Create an AppSorting interface in extensions/browser/ and
make ChromeAppSorting (formerly ExtensionSorting) implement
it.

BUG=313284

Review URL: https://codereview.chromium.org/74613003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236500 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236500