Reset item models in appsGridController dealloc

Also re-enables AppsGridControllerTest.FirstPageKeyboardNavigation

BUG=321284

Review URL: https://codereview.chromium.org/77453008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236789 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236789