Fix crash in InstallWorkerTest.TestInstallChromeSingleSystem.

App command code expects WorkItemList methods to return WorkItem
instances. Update the test to return bogus instances so that the test
doesn't crash.

BUG=153829
NOTRY=true

Review URL: https://codereview.chromium.org/98253006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241323 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241323