drive: Add showroot=true to WAPI feed URLs and ignore "no parent" entries.

chrome/test/data/chromeos/gdata/root_feed.json

 {
    "encoding": "UTF-8",
    "feed": {
       "author": [ {
          "email": {
             "$t": "tester@testing.com"
          },
          "name": {
             "$t": "tester"
          }
@@ skipping 1067 matching lines @@
             "type": "application/atom+xml"
          }, {
             "href": "https://sub_dir_folder_2_self_link/folder:sub_dir_folder_2_self_link",
             "rel": "self",
             "type": "application/atom+xml"
          }, {
             "href": "https://sub_dir_folder_2_self_link/folder:sub_dir_folder_2_self_link",
             "rel": "edit",
             "type": "application/atom+xml"
          }, {
-            "href": "https://slash_dir_self_link",
-            "rel": "http://schemas.google.com/docs/2007#parent",
-            "type": "application/atom+xml"
-         }, {
             "href": "https://fake_server/fake_root",
             "rel": "http://schemas.google.com/docs/2007#parent",
             "type": "application/atom+xml"
          } ],
          "published": {
             "$t": "2010-11-07T05:03:54.719Z"
          },
          "title": {
             "$t": "Directory 2"
          },
@@ skipping 143 matching lines @@
          "$t": "2011-12-14T01:03:21.151Z"
       },
       "xmlns": "http://www.w3.org/2005/Atom",
       "xmlns$batch": "http://schemas.google.com/gdata/batch",
       "xmlns$docs": "http://schemas.google.com/docs/2007",
       "xmlns$gd": "http://schemas.google.com/g/2005",
       "xmlns$openSearch": "http://a9.com/-/spec/opensearch/1.1/"
    },
    "version": "1.0"
 }