More groundwork for flat file based response storage.

More groundwork for filestream based response storage.

This CL touches many files, but it amounts to mindless plumbing of a group_id value.
This doesn't involve any new database or fileio or task scheduling.

In order to read or write a response, callers now need to provide the group_id of
the corresponding manifest, so the AppCacheStorage CreateRepsonseReader() and
CreateResponseWriter() methods now take this additional parameter.
The current disk_cache based impl doesn't use this param for anything, but the
filestream based impl will use it to keep responses for a group in a particular
subdirectory.

When looking up the main resource for a page  load via FindResponseForMainRequest,
the group_id needs to be returned so the response may later be read, the
AppCacheStorage::Delegate OnMainResponseFound() method has a param for that.

A bunch of callsites, tests, and mocks are updated accordingly.

BUG=78359
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=106755

[michaeln, 2011-10-20 02:37:42]

ptal

[rvargas (doing something else), 2011-10-21 01:53:14]

LGTM