Guard native notification code under ENABLE_NATIVE_NOTIFICATIONS guards

about_flags.cc today opt in/out configurations manually, rather
than using the existing ENABLE_NATIVE_NOTIFICATIONS build flag.

This fixes the OZONE/Linux build.

BUG=676220
R=thomasanderson@google.com

Review-Url: https://codereview.chromium.org/2802243002
Cr-Commit-Position: refs/heads/master@{#462896}

chrome/browser/about_flags.cc

@@ -1528,11 +1528,11 @@ const FeatureEntry kFeatureEntries[] = {
     {"translate-lang-by-ulp", flag_descriptions::kTranslateLanguageByUlpName,
      flag_descriptions::kTranslateLanguageByUlpDescription, kOsAll,
      FEATURE_VALUE_TYPE(translate::kTranslateLanguageByULP)},
-#if defined(OS_MACOSX) || (defined(OS_LINUX) && !defined(OS_CHROMEOS))
+#if BUILDFLAG(ENABLE_NATIVE_NOTIFICATIONS)
     {"enable-native-notifications", flag_descriptions::kNotificationsNativeFlag,
      flag_descriptions::kNotificationsNativeFlagDescription, kOsMac | kOsLinux,
      FEATURE_VALUE_TYPE(features::kNativeNotifications)},
-#endif  // defined(OS_MACOSX) || (defined(OS_LINUX) && !defined(OS_CHROMEOS))
+#endif  // ENABLE_NATIVE_NOTIFICATIONS
 #if defined(TOOLKIT_VIEWS)
     {"disable-views-rect-based-targeting",
      flag_descriptions::kViewsRectBasedTargetingName,