drive: Implement deletion of shared entries under MyDrive as unparenting.

BUG=293003

Review URL: https://chromiumcodereview.appspot.com/24288005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@224574 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 224574