[iOS] Add named guide to Discover feed header menu button

Adds a named guide for the Discover feed header menu gear icon. This
guide is meant to be used in the feed IPH message.

Bug: 1124448
Change-Id: If86cc3d5d0d9b3934d135f56a1e3b85954618aa1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391542Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804004}

ios/chrome/browser/ui/content_suggestions/cells/content_suggestions_discover_header_item.mm

@@ -134,6 +134,13 @@ const CGFloat kHeaderChangeAnimationDuration = 0.3;
   } else {
     [self setHeaderForFeedVisible:visible animate:NO];
   }
+  [self.contentView layoutIfNeeded];
+  NamedGuide* menuButtonGuide =
+      [NamedGuide guideWithName:kDiscoverFeedHeaderMenuGuide
+                           view:self.menuButton];
+
+  menuButtonGuide.constrainedFrame =
+      [self.contentView convertRect:self.menuButton.frame toView:nil];
   self.discoverFeedVisible = [NSNumber numberWithBool:visible];
 }