Add changestamp field to google_apis::DocumentEntry

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

 {
    "encoding": "UTF-8",
    "feed": {
       "author": [ {
          "email": {
             "$t": "tester@testing.com"
          },
          "name": {
             "$t": "tester"
          }
@@ skipping 98 matching lines @@
          } ],
          "category": [ {
             "label": "audio/mpeg",
             "scheme": "http://schemas.google.com/g/2005#kind",
             "term": "http://schemas.google.com/docs/2007#file"
          } ],
          "content": {
             "src": "https://file_content_url/",
             "type": "audio/mpeg"
          },
+         "docs$changestamp": {
+            "value": "654321"
+         },
          "docs$filename": {
             "$t": "filename.m4a"
          },
          "docs$md5Checksum": {
             "$t": "3b4382ebefec6e743578c76bbd0575ce"
          },
          "docs$size": {
             "$t": "892721"
          },
          "docs$suggestedFilename": {
@@ skipping 275 matching lines @@
       },
       "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"
 }