Preserve fractional scroll offset for JS scrolling API

Source/platform/geometry/DoubleSize.h

 // Copyright 2014 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 DoubleSize_h
 #define DoubleSize_h
 
 #include "platform/geometry/FloatSize.h"
 #include "platform/geometry/IntSize.h"
 #include "wtf/MathExtras.h"
 
 namespace blink {
 
 class PLATFORM_EXPORT DoubleSize {
 public:
     DoubleSize() : m_width(0), m_height(0) { }
     DoubleSize(double width, double height) : m_width(width), m_height(height) { }
+    DoubleSize(const IntSize& p) : m_width(p.width()), m_height(p.height()) { }
 
     double width() const { return m_width; }
     double height() const { return m_height; }
 
     void setWidth(double width) { m_width = width; }
     void setHeight(double height) { m_height = height; }
 
     bool isEmpty() const { return m_width <= 0 || m_height <= 0; }
 
     bool isZero() const;
 
 private:
     double m_width, m_height;
 };
 
 inline DoubleSize& operator+=(DoubleSize& a, const DoubleSize& b)
 {
     a.setWidth(a.width() + b.width());
     a.setHeight(a.height() + b.height());
     return a;
 }
 
+inline DoubleSize operator+(const DoubleSize& a, const DoubleSize& b)
+{
+    return DoubleSize(a.width() + b.width(), a.height() + b.height());
+}
+
 inline DoubleSize operator-(const DoubleSize& a, const DoubleSize& b)
 {
     return DoubleSize(a.width() - b.width(), a.height() - b.height());
 }
 
 inline bool operator==(const DoubleSize& a, const DoubleSize& b)
 {
     return a.width() == b.width() && a.height() == b.height();
 }
 
+inline bool operator!=(const DoubleSize& a, const DoubleSize& b)
+{
+    return a.width() != b.width() || a.height() != b.height();
+}
+
 inline IntSize flooredIntSize(const DoubleSize& p)
 {
     return IntSize(clampTo<int>(floor(p.width())), clampTo<int>(floor(p.height())));
 }
 
 inline FloatSize toFloatSize(const DoubleSize& p)
 {
     return FloatSize(p.width(), p.height());
 }
 
 } // namespace blink
 
 #endif // DoubleSize_h