Preserve fractional scroll offset for JS scrolling API

Source/platform/geometry/DoublePoint.h

Index: Source/platform/geometry/DoublePoint.h
diff --git a/Source/platform/geometry/DoublePoint.h b/Source/platform/geometry/DoublePoint.h
index 7757161248067de85aa79b62eb6d6d1420b8fec7..f7f79736d38a153ced68640a76b04f31f3c4b15d 100644
--- a/Source/platform/geometry/DoublePoint.h
+++ b/Source/platform/geometry/DoublePoint.h
@@ -23,17 +23,22 @@ public:
         , m_y(y)
     {
     }
-    explicit DoublePoint(const IntPoint& p)
+    DoublePoint(const IntPoint& p)
         : m_x(p.x())
         , m_y(p.y())
     {
     }
-    explicit DoublePoint(const FloatPoint& p)
+    DoublePoint(const FloatPoint& p)
         : m_x(p.x())
         , m_y(p.y())
     {
     }
 
+    explicit DoublePoint(const DoubleSize& size)
+        : m_x(size.width()), m_y(size.height())
+    {
+    }
+
     DoublePoint expandedTo(int x, int y) const
     {
         return DoublePoint(m_x > x ? m_x : x, m_y > y ? m_y : y);
@@ -61,11 +66,21 @@ inline bool operator!=(const DoublePoint& a, const DoublePoint& b)
     return a.x() != b.x() || a.y() != b.y();
 }
 
+inline DoublePoint operator+(const DoublePoint& a, const DoubleSize& b)
+{
+    return DoublePoint(a.x() + b.width(), a.y() + b.height());
+}
+
 inline DoubleSize operator-(const DoublePoint& a, const DoublePoint& b)
 {
     return DoubleSize(a.x() - b.x(), a.y() - b.y());
 }
 
+inline DoublePoint operator-(const DoublePoint& a)
+{
+    return DoublePoint(-a.x(), -a.y());
+}
+
 inline IntPoint flooredIntPoint(const DoublePoint& p)
 {
     return IntPoint(clampToInteger(floor(p.x())), clampToInteger(floor(p.y())));