Merge 180450 "Avoid GCing an aborting and stopped FileReader."

Merge 180450 "Avoid GCing an aborting and stopped FileReader."

> 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

TBR=sigbjornf@opera.com

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=181365

[sof, 2014-09-04 09:51:17]

Committed patchset #1 (id:1) manually as r181365.