Http cache: Extend support for byte range requests. * Now we handle regular requests (not byte range requests) that end up reading a cached entry that stores byte ranges. In this case we create a control object (partial_), but it's byte_range_ member is always invalid because the user is not requesting a range. * Given that we may find stored 206s that are not keeping sparse data, we detect that case and handle it. * Now we may end up reading 206 from disk (and the net) and having to change the returned status to be 200 (for regular requests). * We avoid performing re-validations for each piece of stored data. Instead, we consider the whole entry to be revalidated once, and read from the cache without asking the server (as far as we can). * When processing the received headers we now consider receiving 200 and 416 (instead of 206/304) and we handle inconsistencies in the range returned by the server (from what we expect). We also handle receiving 206 when we don't expect it. BUG=12258 TEST=unittests Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=23123