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

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

 {
    "encoding": "UTF-8",
    "feed": {
       "author": [ {
          "email": {
             "$t": "tester@test.com"
          },
          "name": {
             "$t": "tester"
          }
@@ skipping 90 matching lines @@
             "rel": "edit",
             "type": "application/atom+xml"
          }, {
             "href": "https://edit-media/document%3Aadded_in_root_id",
             "rel": "edit-media",
             "type": "text/html"
          }, {
             "href": "https://changes/16683",
             "rel": "self",
             "type": "application/atom+xml"
+         }, {
+            "href": "https://fake_server/fake_root",
+            "rel": "http://schemas.google.com/docs/2007#parent",
+            "type": "application/atom+xml"
          } ],
          "published": {
             "$t": "2012-04-10T22:50:53.237Z"
          },
          "title": {
             "$t": "Added file"
          },
          "updated": {
             "$t": "2012-04-10T22:50:55.797Z"
          }
@@ skipping 29 matching lines @@
       "updated": {
          "$t": "2012-04-10T22:50:55.797Z"
       },
       "xmlns": "http://www.w3.org/2005/Atom",
       "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"
 }