Don't pass SDCH GetDictionary event for cached responses

net/url_request/url_request_http_job.cc

Index: net/url_request/url_request_http_job.cc
diff --git a/net/url_request/url_request_http_job.cc b/net/url_request/url_request_http_job.cc
index 47a0666e3fc7ad50225110e7f394da8adf08c83c..9a051701a6939c98e47e62b87ba39ecffdab81ec 100644
--- a/net/url_request/url_request_http_job.cc
+++ b/net/url_request/url_request_http_job.cc
@@ -366,7 +366,9 @@ void URLRequestHttpJob::NotifyHeadersComplete() {
       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) {