Reland "Turn on native notifications by default for Chrome OS."

This is a reland of 8ce1863f

The last one was reverted for making some tests flaky. That should
be fixed by 661500f0

Original change's description:
> Turn on native notifications by default for Chrome OS.
>
> They're already on by default for Mash. This makes Chrome use the mojo
> ("native") interface for notifications even in non-Mash.
>
> Bug: 578868
> Change-Id: If1ee85e2136dc0480ccee9a57835b0ffb9b47b8b
> Reviewed-on: https://chromium-review.googlesource.com/963518
> Commit-Queue: Evan Stade <estade@chromium.org>
> Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#544395}

Bug: 578868
Change-Id: I33f5ec5a614e2cd7f0c745944413ea49965b82bf
Reviewed-on: https://chromium-review.googlesource.com/999333Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548796}

chrome/common/chrome_features.cc

@@ -407,10 +407,9 @@ const base::Feature kMultiDeviceApi{"MultiDeviceApi",
 #endif
 
 // Enables the use of native notification centers instead of using the Message
-// Center for displaying the toasts.
+// Center for displaying the toasts. Note that OS_LINUX includes Chrome OS.
 #if BUILDFLAG(ENABLE_NATIVE_NOTIFICATIONS)
-#if defined(OS_MACOSX) || defined(OS_ANDROID) || \
-    (defined(OS_LINUX) && !defined(OS_CHROMEOS))
+#if defined(OS_MACOSX) || defined(OS_ANDROID) || defined(OS_LINUX)
 const base::Feature kNativeNotifications{"NativeNotifications",
                                          base::FEATURE_ENABLED_BY_DEFAULT};
 #else