Add a test for cancelling jobs in drive::JobScheduler.

This CL adds a test that cancels jobs that are not yet passed to the Drive
service. Tests for running jobs will come later.

BUG=251104

Review URL: https://codereview.chromium.org/27290002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228855 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228855