Remove GetWebNotificationManager from Platform

After the removal of the WebNotificationManager class, Blink platforms
don't need to provide the GetWebNotificationManager function anymore.

Change-Id: I022c64cba92a03b2cf4c40a1bcfe81b430543e45
Reviewed-on: https://chromium-review.googlesource.com/1061355
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561084}

AUTHORS

@@ -740,6 +740,7 @@ Sayan Nayak <sayan.nayak@samsung.com>
 Scott Blomquist <sblom@microsoft.com>
 Scott D Phillips <scott.d.phillips@intel.com>
 Sean Bryant <sean@cyberwang.net>
+Sebastian Amend <sebastian.amend@googlemail.com>
 Seo Sanghyeon <sanxiyn@gmail.com>
 Seokju Kwon <seokju.kwon@gmail.com>
 SeongTae Jeong <ferendevelop.gl@gmail.com>

third_party/blink/public/platform/platform.h

@@ -111,7 +111,6 @@ class WebMediaStream;
 class WebMediaStreamCenter;
 class WebMediaStreamCenterClient;
 class WebMediaStreamTrack;
-class WebNotificationManager;
 class WebPluginListBuilder;
 class WebPrescientNetworking;
 class WebPublicSuffixList;
@@ -678,12 +677,6 @@ class BLINK_PLATFORM_EXPORT Platform {
 
   virtual WebDatabaseObserver* DatabaseObserver() { return nullptr; }
 
-  // Web Notifications --------------------------------------------------
-
-  virtual WebNotificationManager* GetWebNotificationManager() {
-    return nullptr;
-  }
-
   // Push API------------------------------------------------------------
 
   virtual WebPushProvider* PushProvider() { return nullptr; }