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

This reverts commit b50b951d.

Reason for revert: suspect of causing crashes in tests, e.g.
https://ci.chromium.org/p/chromium/builders/ci/Mac%20ASan%2064%20Tests%20%281%29/60586

Original change's description:
> [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/+/2036483
> Reviewed-by: Connie Wan <connily@chromium.org>
> Commit-Queue: Charlene Yan <cyan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#738308}

TBR=cyan@chromium.org,connily@chromium.org

Change-Id: I586becb5914e17465a8849d6aa93c73b77c2fdd3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1046554
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2035896Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738539}

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

@@ -607,16 +607,10 @@ 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 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;
-    }
+  // If the adjacent view is actually a group header, show the separator since
+  // the group header takes up a slot.
+  if (adjacent_to_header)
     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.