FileReader.readystate should be LOADING in progress event

BUG=275087
TEST=fast/files/file-reader-readystate.html
TEST=https://github.com/onlimii/web-platform-tests/blob/submission/jcxia-fileapi-filereader/FileAPI/FileReader-interface/filereader_readAsText.html

Review URL: https://chromiumcodereview.appspot.com/22980019

git-svn-id: svn://svn.chromium.org/blink/trunk@156467 bbb929c8-8fbe-4397-9dbb-9b2b20218538