The tentative SDCH filter (SourceStream::TYPE_SDCH_POSSIBLE) should pass through raw contents unchanged when decoding fails, but right now it issues a meta-refresh.

Details:
When URLRequestHttpJob has sent "Avail-Dictionary" but received a response with only "Content-Encoding: gzip", URLRequestHttpJob adds a tentative SDCH filter to the filter chain. This hack was added to work around the fact that some proxies strip out SDCH encoding header.

When decoding a gzipped resource, SdchSourceStream will fail to find a valid SDCH dictionary id in the response. SdchSourceStream then calls SdchPolicyDelegate::OnDictionaryIdError(). If |possible_pass_through_| (true for tentative Sdch filter), we should pass through the contents unchanged, but we issue a meta-refresh instead. 
This is found during a refactoring CL (ab5a1f3c40b48b2a0498e6469f9f8d676a728994). We left the old logic unchanged. 

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SdchPolicyDelegate::ErrorRecovery SdchPolicyDelegate::OnDictionaryIdError(
    std::string* replace_output) {
  if (possible_pass_through_) {
    LogCorruptionDetection(net_log_, is_cached_content_,
                           RESPONSE_TENTATIVE_SDCH);
    // Ideally we should return PASS_THROUGH here, but this is done to match
    // the old behavior in sdch_filter.cc.
  }
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