Set track_start on solid color layers to the margin.

Source/core/page/scrolling/ScrollingCoordinator.cpp

Index: Source/core/page/scrolling/ScrollingCoordinator.cpp
diff --git a/Source/core/page/scrolling/ScrollingCoordinator.cpp b/Source/core/page/scrolling/ScrollingCoordinator.cpp
index ba8cca0f7d5d8ef99e1b655de4918b0e15d4b823..453a30eb3d42c44545c5317b93adb296b9f453f1 100644
--- a/Source/core/page/scrolling/ScrollingCoordinator.cpp
+++ b/Source/core/page/scrolling/ScrollingCoordinator.cpp
@@ -247,10 +247,10 @@ static PassOwnPtr<WebScrollbarLayer> createScrollbarLayer(Scrollbar* scrollbar)
     return scrollbarLayer.release();
 }
 
-PassOwnPtr<WebScrollbarLayer> ScrollingCoordinator::createSolidColorScrollbarLayer(ScrollbarOrientation orientation, int thumbThickness, bool isLeftSideVerticalScrollbar)
+PassOwnPtr<WebScrollbarLayer> ScrollingCoordinator::createSolidColorScrollbarLayer(ScrollbarOrientation orientation, int thumbThickness, int trackStart, bool isLeftSideVerticalScrollbar)
 {
     blink::WebScrollbar::Orientation webOrientation = (orientation == HorizontalScrollbar) ? blink::WebScrollbar::Horizontal : blink::WebScrollbar::Vertical;
-    OwnPtr<WebScrollbarLayer> scrollbarLayer = adoptPtr(blink::Platform::current()->compositorSupport()->createSolidColorScrollbarLayer(webOrientation, thumbThickness, isLeftSideVerticalScrollbar));
+    OwnPtr<WebScrollbarLayer> scrollbarLayer = adoptPtr(blink::Platform::current()->compositorSupport()->createSolidColorScrollbarLayer(webOrientation, thumbThickness, trackStart, isLeftSideVerticalScrollbar));
     GraphicsLayer::registerContentsLayer(scrollbarLayer->layer());
     return scrollbarLayer.release();
 }
@@ -328,7 +328,7 @@ void ScrollingCoordinator::scrollableAreaScrollbarLayerDidChange(ScrollableArea*
             OwnPtr<WebScrollbarLayer> webScrollbarLayer;
             if (settings->useSolidColorScrollbars()) {
                 ASSERT(RuntimeEnabledFeatures::overlayScrollbarsEnabled());
-                webScrollbarLayer = createSolidColorScrollbarLayer(orientation, scrollbar->theme()->thumbThickness(scrollbar), scrollableArea->shouldPlaceVerticalScrollbarOnLeft());
+                webScrollbarLayer = createSolidColorScrollbarLayer(orientation, scrollbar->theme()->thumbThickness(scrollbar), scrollbar->theme()->trackPosition(scrollbar), scrollableArea->shouldPlaceVerticalScrollbarOnLeft());
             } else {
                 webScrollbarLayer = createScrollbarLayer(scrollbar);
             }