[omnibox] Enable on-device model for mobile Incognito by default.

Bug: 1000444
Change-Id: I9a4261f503999c89e5f53da7110b15deecbe3cba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2523239
Commit-Queue: Justin Donnelly <jdonnelly@chromium.org>
Reviewed-by: manuk hovanesian <manukh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825066}

components/omnibox/browser/on_device_head_provider_unittest.cc

@@ -121,12 +121,17 @@ TEST_F(OnDeviceHeadProviderTest, TestIfIncognitoIsAllowed) {
   EXPECT_CALL(*client_.get(), SearchSuggestEnabled())
       .WillRepeatedly(Return(true));
 
-  // By default incognito request will be rejected.
+  // By default incognito request will be accepted on mobile, rejected on
+  // desktop.
   {
     base::test::ScopedFeatureList scoped_feature_list;
     scoped_feature_list.InitAndEnableFeature(
         omnibox::kOnDeviceHeadProviderNonIncognito);
+#if defined(OS_ANDROID) || defined(OS_IOS)
+    ASSERT_TRUE(IsOnDeviceHeadProviderAllowed(input));
+#else
     ASSERT_FALSE(IsOnDeviceHeadProviderAllowed(input));
+#endif
   }
 
   // Now enable for incognito.

components/omnibox/common/omnibox_features.cc

@@ -251,7 +251,7 @@ const base::Feature kReactiveZeroSuggestionsOnNTPRealbox{
 // NonIncognito will only controls behaviors under incognito / non-incognito
 // mode respectively.
 const base::Feature kOnDeviceHeadProviderIncognito{
-    "OmniboxOnDeviceHeadProviderIncognito", base::FEATURE_DISABLED_BY_DEFAULT};
+    "OmniboxOnDeviceHeadProviderIncognito", enabled_by_default_android_ios};
 const base::Feature kOnDeviceHeadProviderNonIncognito{
     "OmniboxOnDeviceHeadProviderNonIncognito", enabled_by_default_android_ios};