Enable Local-history ZPS for signed-out users by default.

This change enables on-device history ZPS suggestions for users
who are not signed in to Chrome on Android.

Bug: 956837
Change-Id: I689960538b1087eac9355ba0b20da7310f5e3daa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2132864
Commit-Queue: Ender <ender@google.com>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#755854}

components/omnibox/browser/local_history_zero_suggest_provider.cc

@@ -48,6 +48,19 @@ base::string16 GetSearchTermsFromURL(const GURL& url,
   return base::i18n::ToLower(base::CollapseWhitespace(search_terms, false));
 }
 
+// Whether zero suggest suggestions are allowed in the given context.
+// Invoked early, confirms all the conditions for zero suggestions are met.
+bool AllowLocalHistoryZeroSuggestSuggestions(const AutocompleteInput& input) {
+#if defined(OS_ANDROID)  // Default-enabled on Android.
+  return true;
+#else
+  return base::Contains(
+      OmniboxFieldTrial::GetZeroSuggestVariants(
+          input.current_page_classification()),
+      LocalHistoryZeroSuggestProvider::kZeroSuggestLocalVariant);
+#endif
+}
+
 }  // namespace
 
 // static
@@ -91,11 +104,8 @@ void LocalHistoryZeroSuggestProvider::Start(const AutocompleteInput& input,
     return;
   }
 
-  if (!base::Contains(OmniboxFieldTrial::GetZeroSuggestVariants(
-                          input.current_page_classification()),
-                      kZeroSuggestLocalVariant)) {
+  if (!AllowLocalHistoryZeroSuggestSuggestions(input))
     return;
-  }
 
   QueryURLDatabase(input);
 }

components/omnibox/browser/local_history_zero_suggest_provider_unittest.cc

@@ -291,7 +291,11 @@ TEST_F(LocalHistoryZeroSuggestProviderTest, ZeroSuggestVariant) {
   SetZeroSuggestVariant(metrics::OmniboxEventProto::NTP_REALBOX,
                         /*zero_suggest_variant_value=*/"blah");
   StartProviderAndWaitUntilDone();
+#if defined(OS_ANDROID)  // Enabled by default.
+  ExpectMatches({{"hello world", 500}});
+#else
   ExpectMatches({});
+#endif
 
   SetZeroSuggestVariant(
       metrics::OmniboxEventProto::NTP_REALBOX,