Scale scrollbar in use-zoom-for-dsf mode

third_party/WebKit/Source/platform/scroll/Scrollbar.cpp

Index: third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp b/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
index f79cdd89d63dae424a9ad66b06d9d03829f2887c..4fc5850f148d821955e40fd30febf64aa14262d0 100644
--- a/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
+++ b/third_party/WebKit/Source/platform/scroll/Scrollbar.cpp
@@ -26,6 +26,7 @@
 #include "platform/scroll/Scrollbar.h"
 
 #include <algorithm>
+#include "platform/HostWindow.h"
 #include "platform/PlatformGestureEvent.h"
 #include "platform/PlatformMouseEvent.h"
 #include "platform/graphics/paint/CullRect.h"
@@ -40,17 +41,17 @@
 
 namespace blink {
 
-PassRefPtrWillBeRawPtr<Scrollbar> Scrollbar::create(ScrollableArea* scrollableArea, ScrollbarOrientation orientation, ScrollbarControlSize size)
+PassRefPtrWillBeRawPtr<Scrollbar> Scrollbar::create(ScrollableArea* scrollableArea, ScrollbarOrientation orientation, ScrollbarControlSize size, HostWindow* hostWindow)
 {
-    return adoptRefWillBeNoop(new Scrollbar(scrollableArea, orientation, size));
+    return adoptRefWillBeNoop(new Scrollbar(scrollableArea, orientation, size, hostWindow));
 }
 
 PassRefPtrWillBeRawPtr<Scrollbar> Scrollbar::createForTesting(ScrollableArea* scrollableArea, ScrollbarOrientation orientation, ScrollbarControlSize size, ScrollbarTheme* theme)
 {
-    return adoptRefWillBeNoop(new Scrollbar(scrollableArea, orientation, size, theme));
+    return adoptRefWillBeNoop(new Scrollbar(scrollableArea, orientation, size, nullptr, theme));
 }
 
-Scrollbar::Scrollbar(ScrollableArea* scrollableArea, ScrollbarOrientation orientation, ScrollbarControlSize controlSize, ScrollbarTheme* theme)
+Scrollbar::Scrollbar(ScrollableArea* scrollableArea, ScrollbarOrientation orientation, ScrollbarControlSize controlSize, HostWindow* hostWindow, ScrollbarTheme* theme)
     : m_scrollableArea(scrollableArea)
     , m_orientation(orientation)
     , m_controlSize(controlSize)
@@ -78,6 +79,15 @@ Scrollbar::Scrollbar(ScrollableArea* scrollableArea, ScrollbarOrientation orient
     // scrollbar thickness and use it when sizing scrollbars (rather than leaving one dimension of the scrollbar
     // alone when sizing).
     int thickness = m_theme.scrollbarThickness(controlSize);
+    if (hostWindow) {
+        // Convert the thickness in the screen (dip) to the viewport
+        // by applying the invert of the viewport to screen scale.
+        // TODO(oshima): Introduce the screenToViewport(float) to avoid the rounding error.
+        IntRect inScreen(0, 0, 100, 0);
+        inScreen = hostWindow->viewportToScreen(inScreen);
+        thickness *= 100.f / inScreen.width();
+    }
+
     Widget::setFrameRect(IntRect(0, 0, thickness, thickness));
 
     m_currentPos = scrollableAreaCurrentPos();