Fix incorrect use of AppListItemList::FindItem

This CL should:
* Fix a Folders bugs where existing items in folders were not found (causing them not to be updated and potentially creating duplicate items)
* Protect the code against similar errors in the future
* Correctly protect against an edge case where multiple items might have the same ordianl

BUG=350165, 346117
For sync_app_list_helper.cc:

R=jennyz@chromium.org, tapted@chromium.org
TBR=zea@chromium.org

Review URL: https://codereview.chromium.org/190953012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256603 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256603