Add non-member non-mutating methods for common gfx::Rect operations.

ui/gfx/rect_f.cc

Index: ui/gfx/rect_f.cc
diff --git a/ui/gfx/rect_f.cc b/ui/gfx/rect_f.cc
index 85039244789099ed8f6922e520a31469ea7d6f24..6035391cff9f129997eb3933a08892f21a766b02 100644
--- a/ui/gfx/rect_f.cc
+++ b/ui/gfx/rect_f.cc
@@ -42,4 +42,28 @@ std::string RectF::ToString() const {
                             size().ToString().c_str());
 }
 
+RectF IntersectRects(const RectF& a, const RectF& b) {
+  RectF result = a;
+  result.Intersect(b);
+  return result;
+}
+
+RectF UnionRects(const RectF& a, const RectF& b) {
+  RectF result = a;
+  result.Union(b);
+  return result;
+}
+
+RectF SubtractRects(const RectF& a, const RectF& b) {
+  RectF result = a;
+  result.Subtract(b);
+  return result;
+}
+
+RectF ScaleRect(const RectF& r, float x_scale, float y_scale) {
+  RectF result = r;
+  result.Scale(x_scale, y_scale);
+  return result;
+}
+
 }  // namespace gfx