Fix group header drag animation

See bug for screencast of issue.

The issue only seems to be present on Mac, and I tested dragging on both Mac and Windows with this change.

Bug: 1033534
Change-Id: I10553e4121f1e3566f66ffd215cfd276ac8ab801
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2261222Reviewed-by: Charlene Yan <cyan@chromium.org>
Commit-Queue: Charlene Yan <cyan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781542}

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

@@ -265,11 +265,8 @@ int TabStripLayoutHelper::UpdateIdealBounds(int available_width) {
         }
         break;
       case ViewType::kGroupHeader:
-        if (slot.view->dragging()) {
-          slot.view->SetBoundsRect(bounds[i]);
-        } else {
+        if (!slot.view->dragging())
           group_header_ideal_bounds_[slot.view->group().value()] = bounds[i];
-        }
         break;
     }
   }