Make compositor and blink talk in fractional scroll offset

Source/platform/graphics/GraphicsLayerTest.cpp

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *     notice, this list of conditions and the following disclaimer in the
@@ skipping 128 matching lines @@
     virtual bool userInputScrollable(ScrollbarOrientation) const override { return true; }
     virtual bool shouldPlaceVerticalScrollbarOnLeft() const override { return false; }
     virtual int pageStep(ScrollbarOrientation) const override { return 0; }
     virtual IntPoint minimumScrollPosition() const override { return IntPoint(); }
     virtual IntPoint maximumScrollPosition() const override
     {
         return IntPoint(contentsSize().width() - visibleWidth(), contentsSize().height() - visibleHeight());
     }
 
     virtual void setScrollOffset(const IntPoint& scrollOffset) override { m_scrollPosition = scrollOffset; }
-    virtual IntPoint scrollPosition() const override { return m_scrollPosition; }
+    virtual void setScrollOffset(const DoublePoint& scrollOffset) override { m_scrollPosition = scrollOffset; }
+    virtual DoublePoint scrollPositionDouble() const override { return m_scrollPosition; }
+    virtual IntPoint scrollPosition() const override { return flooredIntPoint(m_scrollPosition); }
 
 private:
-    IntPoint m_scrollPosition;
+    DoublePoint m_scrollPosition;
 };
 
 TEST_F(GraphicsLayerTest, applyScrollToScrollableArea)
 {
     FakeScrollableArea scrollableArea;
     m_graphicsLayer->setScrollableArea(&scrollableArea, false);
 
-    WebPoint scrollPosition(7, 9);
-    m_platformLayer->setScrollPosition(scrollPosition);
+    WebDoublePoint scrollPosition(7.2, 9.6);
+    m_platformLayer->setScrollPositionDouble(scrollPosition);
     m_graphicsLayer->didScroll();
 
-    EXPECT_EQ(scrollPosition, WebPoint(scrollableArea.scrollPosition()));
+    EXPECT_FLOAT_EQ(scrollPosition.x, scrollableArea.scrollPositionDouble().x());
+    EXPECT_FLOAT_EQ(scrollPosition.y, scrollableArea.scrollPositionDouble().y());
 }
 
 } // namespace