gdata: Define the resource ID for the root directory

Per the spec, the resource ID for the root directory is defined
as "folder:root". Add the resource ID to the root directory in our
file system representation so we can look up the root directory by
the resource ID.

BUG=127697
TEST=add unit tests

Review URL: https://chromiumcodereview.appspot.com/10332253

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@137928 0039d316-1c4b-4281-b951-d872f2087c98
  void VerifyGetCacheState(base::PlatformFileError error, int cache_state) {
    ++num_callback_invocations_;

    EXPECT_EQ(expected_error_, error);

    if (error == base::PLATFORM_FILE_OK) {
      EXPECT_EQ(expected_cache_state_, cache_state);
    }
  }

  void VerifyGetCacheState(base::PlatformFileError error, int cache_state) {
    ++num_callback_invocations_;

    EXPECT_EQ(expected_error_, error);

    if (error == base::PLATFORM_FILE_OK) {
      EXPECT_EQ(expected_cache_state_, cache_state);
    }
  }
