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 TestRemoveFromCache(const std::string& resource_id,
                           base::PlatformFileError expected_error) {
    expected_error_ = expected_error;

    file_system_->RemoveFromCache(resource_id,
        base::Bind(&GDataFileSystemTest::VerifyRemoveFromCache,
                   base::Unretained(this)));

    RunAllPendingForIO();
  }

  void TestRemoveFromCache(const std::string& resource_id,
                           base::PlatformFileError expected_error) {
    expected_error_ = expected_error;

    file_system_->RemoveFromCache(resource_id,
        base::Bind(&GDataFileSystemTest::VerifyRemoveFromCache,
                   base::Unretained(this)));

    RunAllPendingForIO();
  }
