Add comment to note we should change FileReader::m_loadingState before firing events

A follow-up patch to https://chromiumcodereview.appspot.com/23745005/

BUG=237800
TEST=none

Review URL: https://chromiumcodereview.appspot.com/23545021

git-svn-id: svn://svn.chromium.org/blink/trunk@157126 bbb929c8-8fbe-4397-9dbb-9b2b20218538