Remove wrong ASSERT on request instance for fallback in DocumentThreadableLoader

Remove wrong ASSERT on request instance for fallback in DocumentThreadableLoader

It's possible that the browser process skips the ServiceWorker and complete
request and pass a response with wasFetchedViaServiceWorker set to false to
the DocumentThreadableLoader. It's not right to put ASSERT here.

This ASSERT was introduced by me on
https://src.chromium.org/viewvc/blink?revision=184424&view=revision

BUG=none

[tyoshino (SeeGerritForStatus), 2015-01-09 10:00:03]

tyoshino@chromium.org changed reviewers:
+ horo@chromium.org

[tyoshino (SeeGerritForStatus), 2015-01-09 10:01:00]

Investigating if we can write some simpler test to cover this. But could you
please start reviewing?

[mlamouri (slow - plz ping), 2015-01-12 10:27:54]

On 2015/01/09 at 10:01:00, tyoshino wrote:
> Investigating if we can write some simpler test to cover this. But could you
please start reviewing?

Any ETA for landing this? It is unfortunately blocking
https://codereview.chromium.org/840553003 :(

[horo, 2015-01-13 03:55:37]

On 2015/01/12 10:27:54, Mounir Lamouri wrote:
> On 2015/01/09 at 10:01:00, tyoshino wrote:
> > Investigating if we can write some simpler test to cover this. But could you
> please start reviewing?
> 
> Any ETA for landing this? It is unfortunately blocking
> https://codereview.chromium.org/840553003 :(


I think we should not remove this ASSERT.
See: https://codereview.chromium.org/840553003#msg24

[tyoshino (SeeGerritForStatus), 2015-01-13 05:51:08]

FTR, here's what we've discussed:

- ResourceDispatcherHostImpl::BeingRequest() makes RESOURCE_TYPE_FAVICON
requests skip ServiceWorkers. This is inconsistent with the condition in the
DocumentThreadableLoader (DocumentThreadableLoader is used by
AssociatedURLLoader. DocumentThreadableLoader expects that the browser skips the
ServiceWorker only when skip_service_worker is set or is_sync_load is set), and
so, we hit the ASSERT.

- ServiceWorkerURLRequestJob sets response_type_ to FALLBACK_TO_NETWORK when
request_mode_ is not FETCH_REQUEST_MODE_CORS or
FETCH_REQUEST_MODE_CORS_WITH_FORCED_PREFLIGHT. DocumentThreadableLoader sets
mode to either of them. So, we should never hit this.

- ServiceWorkerControlleeRequestHandler sets response_type_ to
FALLBACK_TO_NETWORK when the ServiceWorker is not ready or stale, or it's not
allowed to forward the request to the ServiceWorker. In such a case, in the
first place, we shouldn't consult the ServiceWorker. If we hit the ASSERT, it
means that something is broken in the ServiceWorker availability checking code.

So, we need to mark the request with skipServiceWorker flag before it goes
through DocumentThreadableLoader.