Call ScrollableArea::ShowOverlayScrollbars for explicit scrolls only.

third_party/WebKit/Source/platform/scroll/ScrollbarTestSuite.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ScrollbarTestSuite_h
 #define ScrollbarTestSuite_h
 
 #include <memory>
 #include "platform/heap/GarbageCollected.h"
 #include "platform/scroll/ScrollableArea.h"
@@ skipping 26 matching lines @@
   MOCK_CONST_METHOD0(IsScrollCornerVisible, bool());
   MOCK_CONST_METHOD0(ScrollCornerRect, IntRect());
   MOCK_CONST_METHOD0(EnclosingScrollableArea, ScrollableArea*());
   MOCK_CONST_METHOD1(VisibleContentRect, IntRect(IncludeScrollbarsInRect));
   MOCK_CONST_METHOD0(ContentsSize, IntSize());
   MOCK_CONST_METHOD0(ScrollableAreaBoundingBox, IntRect());
   MOCK_CONST_METHOD0(LayerForHorizontalScrollbar, GraphicsLayer*());
   MOCK_CONST_METHOD0(LayerForVerticalScrollbar, GraphicsLayer*());
   MOCK_CONST_METHOD0(HorizontalScrollbar, Scrollbar*());
   MOCK_CONST_METHOD0(VerticalScrollbar, Scrollbar*());
+  MOCK_CONST_METHOD0(ScrollbarsHidden, bool());
 
   bool UserInputScrollable(ScrollbarOrientation) const override { return true; }
   bool ScrollbarsCanBeActive() const override { return true; }
   bool ShouldPlaceVerticalScrollbarOnLeft() const override { return false; }
   void UpdateScrollOffset(const ScrollOffset& offset, ScrollType) override {
     scroll_offset_ = offset.ShrunkTo(maximum_scroll_offset_);
   }
   IntSize ScrollOffsetInt() const override {
     return FlooredIntSize(scroll_offset_);
   }
@@ skipping 31 matching lines @@
     maximum_scroll_offset_ = maximum_scroll_offset;
   }
 
   ScrollOffset scroll_offset_;
   ScrollOffset maximum_scroll_offset_;
 };
 
 }  // namespace blink
 
 #endif