Remove the specific error type for a policy change cancellation.

There is very little detail in the code or bug db as to why this is
necessary, but the implication seems to be that this was used
to prevent an assertion failure back in 2007. I can't figure out why we
need this protection still, especially since it's only used when we fail
during redirect or when processing response headers, so it's not like the
plugin isn't ready for a cancellation yet.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/23088012

git-svn-id: svn://svn.chromium.org/blink/trunk@156662 bbb929c8-8fbe-4397-9dbb-9b2b20218538