Revert of Fix cancellation of a pair of URLRequestJob subclasses

Revert of Fix cancellation of a pair of URLRequestJob subclasses (patchset #6 id:240001 of https://codereview.chromium.org/1271593002/ )

Reason for revert:
Caused crashes in the following tests due to DCHECK failures:

http/tests/serviceworker/registration.html
http/tests/serviceworker/chromium/register-error-messages.html

See e.g. http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Mac10.6%20(dbg)/builds/25550/steps/webkit_tests/logs/stdio for a log.

Original issue's description:
> Fix cancellation of a pair of URLRequestJob subclasses
>
> URLRequestSimpleJob and URLRequestChromeJob can call back into their
> parent URLRequestJob class after Kill() has been invoked, which can
> cause Bad Things.
>
> Longer term, we should think about the interface between URLRequestJob
> and its subclasses, to see if we can create a more robust interface.
>
> BUG=508900
>
> Committed: https://crrev.com/363ce98af274c07f43acac74b7ca7c23641c996e
> Cr-Commit-Position: refs/heads/master@{#342211}

TBR=davidben@chromium.org,mmenke@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=508900
Committed: https://crrev.com/857c585526163ae1817352d6e9d805fbd07cb005
Cr-Commit-Position: refs/heads/master@{#342255}

[Peter Kasting, 2015-08-07 01:22:10]

Created Revert of Fix cancellation of a pair of URLRequestJob subclasses

[commit-bot: I haz the power, 2015-08-07 01:23:16]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1273813005/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1273813005/1

[commit-bot: I haz the power, 2015-08-07 01:24:34]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-07 01:25:04]

Patchset 1 (id:??) landed as
https://crrev.com/857c585526163ae1817352d6e9d805fbd07cb005
Cr-Commit-Position: refs/heads/master@{#342255}

[mmenke, 2015-08-07 02:14:18]

Thanks for the revert!