Avoid GCing an aborting and stopped FileReader.

If a FileReader is abort()ed, a task is scheduled to perform the actual
abort operation. Should the containing Document's ActiveDOMObjects be
stopped before that task gets to run, the FileReader would already
advance to a DONE state and be at risk from being GCed before the abort
task gets to run. If so, it would then access a dead object.

The provided test elicits an assert that shows up the problem, but
doesn't trigger the GC and subsequent access of the dead object.

R=kouhei@chromium.org, tzik@chromium.org
BUG=404513

Review URL: https://codereview.chromium.org/478263002

git-svn-id: svn://svn.chromium.org/blink/trunk@180450 bbb929c8-8fbe-4397-9dbb-9b2b20218538