Revert of Ensure that 'fetchMainResource' requests have a frame type. (https://codereview.chromium.org/388253002/)

Reason for revert:
This CL broke the following browser tests in Debug:

ResourceFetcher404
ResourceFetcherDeletedInCallback
ResourceFetcherDidFail
ResourceFetcherDownload
ResourceFetcherPost
ResourceFetcherSetHeader
ResourceFetcherTimeout

SHOULD NEVER BE REACHED
../../third_party/WebKit/Source/platform/exported/WebURLRequest.cpp(495) : static WebURLRequest::TargetType blink::WebURLRequest::targetTypeFromRequestContextAndFrameType(WebURLRequest::RequestContext, WebURLRequest::FrameType)
1   0x7fa3ebdb9efe
2   0x7fa3ebdba02f blink::WebURLRequest::targetType() const
3   0x7fa3e06a1828 content::RenderFrameImpl::willSendRequest(blink::WebLocalFrame*, unsigned int, blink::WebURLRequest&, blink::WebURLResponse const&)
4   0x7fa3e06a22c5 non-virtual thunk to content::RenderFrameImpl::willSendRequest(blink::WebLocalFrame*, unsigned int, blink::WebURLRequest&, blink::WebURLResponse const&)
5   0x7fa3e3d9993d
6   0x7fa3e3e3359d
7   0x7fa3e057351b
8   0x6a427f
9   0x6a660a
10  0x6a7524
11  0x6a74b4
12  0x4ed10e
13  0x7f2b89
14  0x7f36b2
15  0x7f366a
16  0x7f3620
17  0x7fa3ead0abde
18  0x7fa3eaddcb68 base::MessageLoop::RunTask(base::PendingTask const&)
19  0x7fa3eaddce8b base::MessageLoop::DeferOrRunPendingTask(base::PendingTask const&)
20  0x7fa3eaddd0b5 base::MessageLoop::DoWork()
21  0x7fa3eadedf18 base::MessagePumpDefault::Run(base::MessagePump::Delegate*)
22  0x7fa3eaddc4a0 base::MessageLoop::RunHandler()
23  0x7fa3eae44c02 base::RunLoop::Run()
24  0x7fa3eaddbd41 base::MessageLoop::Run()
25  0x7fa3eaeb21f9 base::Thread::Run(base::MessageLoop*)
26  0x7fa3eaeb2433 base::Thread::ThreadMain()
27  0x7fa3eae9d66f
28  0x7fa3db25ee9a
29  0x7fa3daf8b3fd clone


Original issue's description:
> Ensure that 'fetchMainResource' requests have a frame type.
> 
> This patch adds an ASSERT to 'ResourceFetcher::fetchMainResource' which
> ensures that the request's FrameType is not FrameTypeNone, and uses that
> information to ensure that the correct TargetType is synthesized when
> heading back up into Chromium.
> 
> In order to make the ASSERT true, FrameLoader does a bit of work on the
> NavigationAction it creates, and NavigationAction grows a
> 'mutableResourceRequest' method in order to allow that work to happen.
> 
> BUG=390497
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=178181

TBR=jochen@chromium.org,mkwst@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=390497

Review URL: https://codereview.chromium.org/393003002

git-svn-id: svn://svn.chromium.org/blink/trunk@178190 bbb929c8-8fbe-4397-9dbb-9b2b20218538