Revert "[Tab Scrolling] Workaround to hide horizontal scroll bars."

This reverts commit 1f06da32.

Reason for revert: breaks mac and mac is on dogfood. horizontal and
vertical scrollbars appear on mac.

Original change's description:
> [Tab Scrolling] Workaround to hide horizontal scroll bars.
>
> Horizontal Scroll bar visibility is currently coupled with its
> functionality.
>
> Bug: 1122214
> Change-Id: I9a0c3ffb0e7128c4444b175631b7cbaf2411f7af
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2393257
> Commit-Queue: Charlene Yan <cyan@chromium.org>
> Reviewed-by: Peter Boström <pbos@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#804506}

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

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 1122214
Change-Id: I54532dccb5109c3f921b37beb987b22de776be3c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2399513Reviewed-by: Peter Boström <pbos@chromium.org>
Reviewed-by: Charlene Yan <cyan@chromium.org>
Commit-Queue: Charlene Yan <cyan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805368}

chrome/browser/ui/views/frame/tab_strip_region_view.cc

@@ -28,12 +28,7 @@ TabStripRegionView::TabStripRegionView(std::unique_ptr<TabStrip> tab_strip) {
     views::ScrollView* tab_strip_scroll_container =
         AddChildView(std::make_unique<views::ScrollView>());
     tab_strip_scroll_container->SetBackgroundColor(base::nullopt);
-
-    // Temporary fix due to ScrollBar visibility coupling with scrolling.
-    // See https://crbug.com/1122214.
-    tab_strip_scroll_container->horizontal_scroll_bar()->SetPreferredSize(
-        gfx::Size());
-
+    tab_strip_scroll_container->SetHideHorizontalScrollBar(true);
     tab_strip_container_ = tab_strip_scroll_container;
     tab_strip_scroll_container->SetContents(std::move(tab_strip));
   } else {