Revert of Remove ResourceFetcher's access to a real FetchContext at frame detach time

Revert of Remove ResourceFetcher's access to a real FetchContext at frame detach time (patchset #4 id:60001 of https://codereview.chromium.org/1035803002/)

Reason for revert:
Causes an assert failure on imported/web-platform-tests/custom-elements/creating-and-passing-registries/share-registry-import-document.html

ASSERTION FAILED: m_thread == currentThread()
../../third_party/WebKit/Source/platform/Timer.h(160) : bool blink::TimerBase::isActive() const
1   0x7f421ebb9357
2   0x7f422003e6bb
3   0x7f422003f214
4   0x7f4229e9ae37 blink::ThreadTimers::sharedTimerFiredInternal()
5   0x7f4229e9aa65 blink::ThreadTimers::sharedTimerFired()

Example failure: http://build.chromium.org/p/chromium.webkit/builders/WebKit%20Linux%20(dbg)/builds/368

I verified locally that reverting fixes the test.

Original issue's description:
> Remove ResourceFetcher's access to a real FetchContext at frame detach time
> 
> This allows us to remove a whole bunch of frame() null-checks in FrameFetchContext.
> 
> BUG=458222
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=193208

TBR=haraken@chromium.org,mkwst@chromium.org,sigbjornf@opera.com,japhet@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=458222
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=193231

[falken, 2015-04-07 01:03:21]

Created Revert of Remove ResourceFetcher's access to a real FetchContext at
frame detach time

[commit-bot: I haz the power, 2015-04-07 01:03:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1063913002/1

[haraken, 2015-04-07 01:04:00]

LGTM

[commit-bot: I haz the power, 2015-04-07 01:04:03]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=193231