Switch arguments to const Length references for performance

Source/platform/LengthPoint.h

Index: Source/platform/LengthPoint.h
diff --git a/Source/platform/LengthPoint.h b/Source/platform/LengthPoint.h
index 75497a1dcc3447a9f6d0598f6e67c4ef7eb9cc6d..c9677a863951bb111a0883b05d5ccf9e7365094c 100644
--- a/Source/platform/LengthPoint.h
+++ b/Source/platform/LengthPoint.h
@@ -40,7 +40,7 @@ public:
     {
     }
 
-    LengthPoint(Length x, Length y)
+    LengthPoint(const Length& x, const Length& y)
         : m_x(x)
         , m_y(y)
     {
@@ -49,11 +49,11 @@ public:
     bool operator==(const LengthPoint& o) const { return m_x == o.m_x && m_y == o.m_y; }
     bool operator!=(const LengthPoint& o) const { return m_x != o.m_x || m_y != o.m_y; }
 
-    void setX(Length x) { m_x = x; }
-    Length x() const { return m_x; }
+    void setX(const Length& x) { m_x = x; }
+    const Length& x() const { return m_x; }
 
-    void setY(Length y) { m_y = y; }
-    Length y() const { return m_y; }
+    void setY(const Length& y) { m_y = y; }
+    const Length& y() const { return m_y; }
 
 private:
     Length m_x;