Don't pass SDCH GetDictionary event for cached responses

net/url_request/url_request_http_job.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/url_request/url_request_http_job.h"
 
 #include "base/base_switches.h"
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/command_line.h"
@@ skipping 348 matching lines @@
       // seen, but rather wait until we can justify their usefulness.
       // For now, we will only fetch the first dictionary, which will at least
       // require multiple suggestions before we get additional ones for this
       // site. Eventually we should wait until a dictionary is requested
       // several times
       // before we even download it (so that we don't waste memory or
       // bandwidth).
       if (GetResponseHeaders()->EnumerateHeader(&iter, name, &url_text)) {
         // Resolve suggested URL relative to request url.
         GURL sdch_dictionary_url = request_->url().Resolve(url_text);
-        if (sdch_dictionary_url.is_valid()) {
+        // Don't try to download Dictionary for cached responses. It's either
+        // useless or too late.

[Randy Smith (Not in Mondays), 2016/01/26 20:44:10]

nit: If you're willing, I'd like to phrase this comment a bit differently:
"Don't re-notify the existence of a Get-Dictionary header for a cached response;
a previous notification was already sent."

Officially, this is just notifying the manager (and any observers through it)
that we've seen an Get-Dictionary header, it's not actually requesting a
download (which is important because of the potential circular dependency in the
net stack).  Part of me wants to wired the information as to whether it's a
cached response through the notification (because the decision as to what to do
with a Get-Dictionary header really belongs in SdchOwner) but that seems like
way too much trouble when it's fairly clear we don't want to do anything with
it.  If the information is ever needed, we can do that wiring later.

+        if (sdch_dictionary_url.is_valid() && !is_cached_content_) {
           rv = sdch_manager->OnGetDictionary(request_->url(),
                                              sdch_dictionary_url);
           if (rv != SDCH_OK) {
             SdchManager::SdchErrorRecovery(rv);
             request_->net_log().AddEvent(
                 NetLog::TYPE_SDCH_DICTIONARY_ERROR,
                 base::Bind(&NetLogSdchDictionaryFetchProblemCallback, rv,
                            sdch_dictionary_url, false));
           }
         }
@@ skipping 1230 matching lines @@
   return override_response_headers_.get() ?
              override_response_headers_.get() :
              transaction_->GetResponseInfo()->headers.get();
 }
 
 void URLRequestHttpJob::NotifyURLRequestDestroyed() {
   awaiting_callback_ = false;
 }
 
 }  // namespace net