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

ui/gfx/rect.cc

Index: ui/gfx/rect.cc
diff --git a/ui/gfx/rect.cc b/ui/gfx/rect.cc
index b46a9d779c266308fd2243df1bd9c49283ff23da..551e6f66b3cda6260b6a754d22d1f151d5da9eb4 100644
--- a/ui/gfx/rect.cc
+++ b/ui/gfx/rect.cc
@@ -88,4 +88,22 @@ std::string Rect::ToString() const {
                             size().ToString().c_str());
 }
 
+Rect IntersectRects(const Rect& a, const Rect& b) {
+  Rect result = a;
+  result.Intersect(b);
+  return result;
+}
+
+Rect UnionRects(const Rect& a, const Rect& b) {
+  Rect result = a;
+  result.Union(b);
+  return result;
+}
+
+Rect SubtractRects(const Rect& a, const Rect& b) {
+  Rect result = a;
+  result.Subtract(b);
+  return result;
+}
+
 }  // namespace gfx