Calling abort() in the last progress event shouldn't crash

Both doAbort() (called via abort()) and didFinishLoading() call
unsetPendingActivity(this), but calling abort() in didFinishLoading()
shoulldn't cause extra deref.

I introduced this bug in r156467
https://src.chromium.org/viewvc/blink?revision=156467&view=revision

Before r156467 we used to set m_state to DONE in didFinishLoading()
before dispatching progress event, therefore calling abort() there
did nothing.

BUG=237800
TEST=fast/files/file-reader-abort-in-last-progress

Review URL: https://chromiumcodereview.appspot.com/23745005

git-svn-id: svn://svn.chromium.org/blink/trunk@157114 bbb929c8-8fbe-4397-9dbb-9b2b20218538