Fix a long standing leak on cancellation of renderer-initiated URLRequests.

Fix a long standing leak on cancellation of renderer-
initiated URLRequests.  ResourceDispatcher was not calling
peer->OnCompletedRequest on cancellation of async requests,
despite it being expected by WebURLLoaderImpl::Context
(The only ResourceLoaderBridge::Peer).

This shouldn't impact externally visible behavior at all,
other than fixing the leak, because once
WebURLLoaderImpl::Context::Cancel() is called, it will never
call back into WebURLLoaderImpl.

BUG=328092
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=241291

[mmenke, 2013-12-16 22:52:08]

jam:  Are you a good reviewer for this?

Another option would just be to have WebURLLoaderImpl synchronously release the
reference it's holding to itself when it calls cancel(), if the request is
async.  If it weren't for the async requirement, I'd prefer this approach.

[jam, 2013-12-17 00:19:48]

lgtm

[mmenke, 2013-12-17 00:32:51]

On 2013/12/17 00:19:48, jam wrote:
> lgtm

Thanks!

[commit-bot: I haz the power, 2013-12-17 00:35:19]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/mmenke@chromium.org/113893007/20001

[commit-bot: I haz the power, 2013-12-17 05:55:01]

Retried try job too often on win_rel for step(s) browser_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_rel&nu...

[commit-bot: I haz the power, 2013-12-17 13:06:02]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/mmenke@chromium.org/113893007/20001

[commit-bot: I haz the power, 2013-12-17 15:13:38]

Change committed as 241291