[Tab Groups] Hide separator for tab next to a named header.

Bug: 1046554
Change-Id: I79f37516e26aeb0fea7261b20082c7f882485918
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036483Reviewed-by: Connie Wan <connily@chromium.org>
Commit-Queue: Charlene Yan <cyan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738308}

chrome/browser/ui/views/tabs/tab_style_views.cc

@@ -607,10 +607,16 @@ float GM2TabStyle::GetSeparatorOpacity(bool for_layout, bool leading) const {
   // animation. Only hide the separator if it's in the first slot, or in
   // certain cases if the tab has a visible background (see below).
 
-  // If the adjacent view is actually a group header, show the separator since
-  // the group header takes up a slot.
-  if (adjacent_to_header)
+  // If the adjacent view is actually a group header, show the separator unless
+  // it is to the right of a named group header.
+  if (adjacent_to_header) {
+    if (leading && !right_tab->controller()
+                        ->GetGroupTitle(right_tab->group().value())
+                        .empty()) {
+      return 0.0f;
+    }
     return GetHoverInterpolatedSeparatorOpacity(for_layout, nullptr);
+  }
 
   // If the tab has a visible background even when not selected or active, there
   // are additional cases where the separators can be hidden.