Fix filesystem API file_handlers to work for drive on ChromeOS.

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

 {
    "encoding": "UTF-8",
    "feed": {
       "author": [ {
          "email": {
             "$t": "tester@testing.com"
          },
          "name": {
             "$t": "tester"
          }
@@ skipping 260 matching lines @@
                "$t": "tester"
             }
          } ],
          "category": [ {
             "label": "test/ro",
             "scheme": "http://schemas.google.com/g/2005#kind",
             "term": "http://schemas.google.com/docs/2007#file"
          } ],
          "content": {
             "src": "https://xxx/content/file:1_file_resource_id",
-            "type": "test/ro"
+            "type": "application/vnd.mozilla.xul+xml"
          },
          "docs$filename": {
             "$t": "test_file.xul"
          },
          "docs$md5Checksum": {
             "$t": "3b4382ebefec6e743578c76bbd0575ce"
          },
          "docs$size": {
             "$t": "13"
          },
@@ skipping 71 matching lines @@
                "$t": "tester"
             }
          } ],
          "category": [ {
             "label": "test/rw",
             "scheme": "http://schemas.google.com/g/2005#kind",
             "term": "http://schemas.google.com/docs/2007#file"
          } ],
          "content": {
             "src": "https://xxx/content/file:2_file_resource_id/",
-            "type": "test/rw"
+            "type": "image/tiff"
          },
          "docs$filename": {
             "$t": "test_file.tiff"
          },
          "docs$md5Checksum": {
             "$t": "3b4382ebefec6e743578c76bbd0575ce"
          },
          "docs$size": {
             "$t": "13"
          },
@@ skipping 375 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"
 }