Add AppListSpecifics .proto definition and support code

This CL introduces the AppList sync type.

As of patchset #7 this CL depends on https://codereview.chromium.org/106033003/

BUG=313376

Review URL: https://codereview.chromium.org/78773004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240162 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240162