Files.app: Fix a reference of member variable in an error handler.

Previously we access a member variable without this.
This CL fixes it.

BUG=none
TEST=none
R=mtomasz@chromium.org

Review URL: https://codereview.chromium.org/119773002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242019 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242019