Allow category["suggestions"] to be absent.

APIs do not return empty lists, apparently. If a repeated field is
empty, then it will be absent in the JSON. Allow this, but make sure
that the empty category is created first.

Review-Url: https://codereview.chromium.org/2279483002
Cr-Commit-Position: refs/heads/master@{#414402}

components/ntp_snippets/ntp_snippets_fetcher.cc

@@ -537,15 +537,19 @@ bool NTPSnippetsFetcher::JsonToSnippets(const base::Value& parsed,
       for (const auto& v : *categories) {
         int category_id = -1;
         const base::DictionaryValue* category_value = nullptr;
-        const base::ListValue* suggestions = nullptr;
         if (!(v->GetAsDictionary(&category_value) &&
               category_value->GetInteger("id", &category_id) &&
-              (category_id > 0) &&
-              category_value->GetList("suggestions", &suggestions))) {
+              (category_id > 0))) {
           return false;
         }
         Category category = category_factory_->FromRemoteCategory(category_id);
+        const base::ListValue* suggestions = nullptr;
         NTPSnippet::PtrVector* articles = &(*snippets)[category];
+        if (!category_value->GetList("suggestions", &suggestions)) {
+          // Absence of a list of suggestions is treated as an empty list, which
+          // is permissible.
+          continue;
+        }
         if (!AddSnippetsFromListValue(
                 /* content_suggestions_api = */ true, *suggestions, articles)) {
           return false;

components/ntp_snippets/ntp_snippets_fetcher_unittest.cc

@@ -426,6 +426,29 @@ TEST_F(NTPSnippetsContentSuggestionsFetcherTest, ShouldFetchSuccessfully) {
                                        /*count=*/1)));
 }
 
+TEST_F(NTPSnippetsContentSuggestionsFetcherTest, EmptyCategoryIsOK) {
+  const std::string kJsonStr =
+      "{\"categories\" : [{"
+      "  \"id\": 1,"
+      "  \"localizedTitle\": \"Articles for You\""
+      "}]}";
+  SetFakeResponse(/*data=*/kJsonStr, net::HTTP_OK,
+                  net::URLRequestStatus::SUCCESS);
+  EXPECT_CALL(mock_callback(), Run(IsEmptyArticleList()));
+  snippets_fetcher().FetchSnippetsFromHosts(test_hosts(), test_lang(),
+                                            /*count=*/1,
+                                            /*interactive_request=*/true);
+  FastForwardUntilNoTasksRemain();
+  EXPECT_THAT(snippets_fetcher().last_status(), Eq("OK"));
+  EXPECT_THAT(snippets_fetcher().last_json(), Eq(kJsonStr));
+  EXPECT_THAT(histogram_tester().GetAllSamples(
+                  "NewTabPage.Snippets.FetchHttpResponseOrErrorCode"),
+              ElementsAre(base::Bucket(/*min=*/200, /*count=*/1)));
+  EXPECT_THAT(histogram_tester().GetAllSamples("NewTabPage.Snippets.FetchTime"),
+              ElementsAre(base::Bucket(/*min=*/kTestJsonParsingLatencyMs,
+                                       /*count=*/1)));
+}
+
 TEST_F(NTPSnippetsContentSuggestionsFetcherTest, ServerCategories) {
   const std::string kJsonStr =
       "{\"categories\" : [{"