Unsubscribe from NSNotificationCenter when WebContents is destroyed

Due to ref-counted references the NSView is not destroyed immediately
when WebContents is destroyed, so it is not reliable to do this in
the destructor.

brightray/browser/inspectable_web_contents_view_mac.mm

@@ -19,6 +19,7 @@ InspectableWebContentsViewMac::InspectableWebContentsViewMac(InspectableWebConte
 }
 
 InspectableWebContentsViewMac::~InspectableWebContentsViewMac() {
+  [view_ removeObservers];
   CloseDevTools();
 }