Add Vector2d classes that represent offsets, instead of using Point.

ui/gfx/point_base.h

Index: ui/gfx/point_base.h
diff --git a/ui/gfx/point_base.h b/ui/gfx/point_base.h
index 34e32f58035010a3312dd692b4df7ad0436a28f6..acaefe17805df6bbc2aa241805175eedea6c6656 100644
--- a/ui/gfx/point_base.h
+++ b/ui/gfx/point_base.h
@@ -14,7 +14,7 @@
 namespace gfx {
 
 // A point has an x and y coordinate.
-template<typename Class, typename Type>
+template<typename Class, typename Type, typename VectorClass>
 class UI_EXPORT PointBase {
  public:
   Type x() const { return x_; }
@@ -33,17 +33,25 @@ class UI_EXPORT PointBase {
     y_ += delta_y;
   }
 
-  Class Add(const Class& other) const WARN_UNUSED_RESULT {
+  void operator+=(const VectorClass& other) {
+    Offset(other.x(), other.y());
+  }
+
+  void operator-=(const VectorClass& other) {
+    Offset(-other.x(), -other.y());
+  }
+
+  Class Add(const VectorClass& other) const WARN_UNUSED_RESULT {
     const Class* orig = static_cast<const Class*>(this);
     Class copy = *orig;
-    copy.Offset(other.x_, other.y_);
+    copy.Offset(other.x(), other.y());
     return copy;
   }
 
-  Class Subtract(const Class& other) const WARN_UNUSED_RESULT {
+  Class Subtract(const VectorClass& other) const WARN_UNUSED_RESULT {
     const Class* orig = static_cast<const Class*>(this);
     Class copy = *orig;
-    copy.Offset(-other.x_, -other.y_);
+    copy.Offset(-other.x(), -other.y());
     return copy;
   }
 
@@ -55,6 +63,14 @@ class UI_EXPORT PointBase {
     return x_ == 0 && y_ == 0;
   }
 
+  VectorClass OffsetFromOrigin() const {
+    return VectorClass(x_, y_);
+  }
+
+  VectorClass OffsetFrom(const Class& other) const {
+    return VectorClass(x_ - other.x_, y_ - other.y_);
+  }
+
   // A point is less than another point if its y-value is closer
   // to the origin. If the y-values are the same, then point with
   // the x-value closer to the origin is considered less than the