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This is to ensure that we can provide functionality to update the AppCache using the URLLoader.
The plan is to implement the URLLoaderClient part of the update job in a subsequent patch.

This patch has the following changes:
1. We have a new class AppCacheUpdateRequestBase which provides the interface which is
   implemented by the URLRequest and URLLoaderClient subclasses. This interface is used
   by the URLFetcher and the AppCacheUpdateJob.

2. Subclasses AppCacheUpdateURLRequest and AppCacheUpdateURLLoaderRequest. These implement
   the URLRequest and URLLoaderClient portion of the update functionality. The URLRequest
   one is fully functional at this point.

3. I have left most of the reading and updating logic unchanged in URLFetcher unchanged.
   Will revisit this when I start working on the URLLoaderClient functionality.

4. The AppCacheUpdateURLRequest subclass maintains a pointer to the URLFetcher base and
   invokes its methods when it receives a response/redirect notification/data, etc.

BUG=715632

Change-Id: I16497e41aa5dbfab0c06c1ab0fc114945dd279e8
Reviewed-on: https://chromium-review.googlesource.com/592809Reviewed-by: Michael Nordman <michaeln@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Ananta Iyengar <ananta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491147}