Add Min() and Max() convenience functions for 2d point types.

This will return (min(x1,x2), min(y1,y2)) and equivalent max from 2
passed-in (x1,y1) (x2,y2) points

gfx/2d/BaseSize.h

@@ -6,6 +6,7 @@
 #ifndef MOZILLA_GFX_BASESIZE_H_
 #define MOZILLA_GFX_BASESIZE_H_
 
+#include <algorithm>
 #include "mozilla/Attributes.h"
 
 namespace mozilla {
@@ -92,6 +93,16 @@ struct BaseSize {
   Sub operator/(const Sub& aSize) const {
     return Sub(width / aSize.width, height / aSize.height);
   }
+
+  friend Sub Min(const Sub& aA, const Sub& aB) {
+    return Sub(std::min(aA.width, aB.width),
+               std::min(aA.height, aB.height));
+  }
+
+  friend Sub Max(const Sub& aA, const Sub& aB) {
+    return Sub(std::max(aA.width, aB.width),
+               std::max(aA.height, aB.height));
+  }
 };
 
 } // namespace gfx