ScrollableArea::pixelsPerLine should return viewport pixels in use-zoom-for-dsf mode

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

Index: third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp
diff --git a/third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp b/third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp
index 6d5b7a1d6413667c0cf6b8b610cbe2324d75243e..704806e6096c549fcaa01e001b38e1e1e2a79205 100644
--- a/third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp
+++ b/third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp
@@ -60,9 +60,12 @@ struct SameSizeAsScrollableArea {
 
 static_assert(sizeof(ScrollableArea) == sizeof(SameSizeAsScrollableArea), "ScrollableArea should stay small");
 
-int ScrollableArea::pixelsPerLineStep()
+int ScrollableArea::pixelsPerLineStep(HostWindow* host)
 {
-    return kPixelsPerLineStep;
+    if (!host)
+        return kPixelsPerLineStep;
+    FloatRect tmpRectInViewport = host->windowToViewport(FloatRect(1, 0, 0, 0));
+    return kPixelsPerLineStep * tmpRectInViewport.x();
 }
 
 float ScrollableArea::minFractionToStepWhenPaging()
@@ -566,7 +569,7 @@ DoublePoint ScrollableArea::clampScrollPosition(const DoublePoint& scrollPositio
 
 int ScrollableArea::lineStep(ScrollbarOrientation) const
 {
-    return pixelsPerLineStep();
+    return pixelsPerLineStep(hostWindow());
 }
 
 int ScrollableArea::pageStep(ScrollbarOrientation orientation) const