Make ScrollBarView default constructible

Use default argument true for creating a horizontal ScrollBarViews by
default.

Bug: 1108460
Change-Id: Ic19c4e34e6c06372d9004ccbb9d17cc72d31c9cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2427525Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810298}

ui/views/controls/scrollbar/scroll_bar_views.h

@@ -25,7 +25,7 @@ class VIEWS_EXPORT ScrollBarViews : public ScrollBar, public ButtonListener {
   METADATA_HEADER(ScrollBarViews);
 
   // Creates new scrollbar, either horizontal or vertical.
-  explicit ScrollBarViews(bool horizontal);
+  explicit ScrollBarViews(bool horizontal = true);
   ~ScrollBarViews() override;
 
   static int GetVerticalScrollBarWidth(const ui::NativeTheme* theme);

ui/views/controls/scrollbar/scrollbar_unittest.cc

@@ -86,8 +86,7 @@ class ScrollBarViewsTest : public ViewsTestBase {
     auto* container =
         widget_->SetContentsView(std::make_unique<TestScrollbarContainer>());
 
-    scrollbar_ =
-        container->AddChildView(std::make_unique<ScrollBarViews>(true));
+    scrollbar_ = container->AddChildView(std::make_unique<ScrollBarViews>());
     scrollbar_->SetBounds(0, 0, 100, 100);
     scrollbar_->Update(100, 1000, 0);
     scrollbar_->set_controller(controller_.get());