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This patch builds on patch https://codereview.chromium.org/2956373002/ for adding fallback
support for subresource requests. This is implemented by passing a proxy URLLoaderClient
interface to the network URL service for subresource requests which are sent to the network.
We intercept the OnReceiveResponse, OnReceiveRedirect and OnComplete calls for supporting
fallback responses in these cases. We could potentially look at moving fallback response
reading to the renderer in a later patchset.

The changes in this patchset are as below:
1. Add support for passing the ResourceResponseHead to the AppCacheURLLoaderJob. This is
   required for supporting fallback responses. The request handler looks at headers in
   the response and the status code to make a call whether a fallback is required.

2. AppCacheRequestHandler::MaybeLoadFallbackForResponse() and MaybeLoadFallbackForRedirect() have
   changed to allow for the fact that they are called in the network service code by the
   AppCacheURLLoaderJob. We don't create new job for delivering the fallback in this case.

3. Added methods in the AppCacheRequest class  to return the URLRequest override and URLLoader
   override respectively.

Next step is to add support for main resource fallback.

BUG=715632

Review-Url: https://codereview.chromium.org/2974733002
Cr-Commit-Position: refs/heads/master@{#487640}