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

This reverts commit 8ce1863f.

Reason for revert: speculative revert to fix CrOS MSan build, see crbug.com/824097.

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}

TBR=stevenjb@chromium.org,estade@chromium.org

Change-Id: I044bbe505034ccb6da615163f6cbe74e7b164f6a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 578868
Reviewed-on: https://chromium-review.googlesource.com/973006Reviewed-by: Alexander Potapenko <glider@chromium.org>
Commit-Queue: Alexander Potapenko <glider@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544664}

chrome/common/chrome_features.cc

@@ -410,9 +410,10 @@ const base::Feature kMultiDeviceApi{"MultiDeviceApi",
 #endif
 
 // Enables the use of native notification centers instead of using the Message
-// Center for displaying the toasts. Note that OS_LINUX includes Chrome OS.
+// Center for displaying the toasts.
 #if BUILDFLAG(ENABLE_NATIVE_NOTIFICATIONS)
-#if defined(OS_MACOSX) || defined(OS_ANDROID) || defined(OS_LINUX)
+#if defined(OS_MACOSX) || defined(OS_ANDROID) || \
+    (defined(OS_LINUX) && !defined(OS_CHROMEOS))
 const base::Feature kNativeNotifications{"NativeNotifications",
                                          base::FEATURE_ENABLED_BY_DEFAULT};
 #else