Fixup various overflow conditions

core/fxcrt/include/fx_coordinates.h

Index: core/fxcrt/include/fx_coordinates.h
diff --git a/core/fxcrt/include/fx_coordinates.h b/core/fxcrt/include/fx_coordinates.h
index eff2a7258a2bd5ca37315894b3d0f1072a5be482..ce97f6f6f3503f3b89240cd5370b4fb993e41014 100644
--- a/core/fxcrt/include/fx_coordinates.h
+++ b/core/fxcrt/include/fx_coordinates.h
@@ -150,6 +150,14 @@ struct FX_RECT {
   int Height() const { return bottom - top; }
   bool IsEmpty() const { return right <= left || bottom <= top; }
 
+  bool Valid() const {
+    pdfium::base::CheckedNumeric<int> w = right;
+    pdfium::base::CheckedNumeric<int> h = bottom;
+    w -= left;
+    h -= top;
+    return w.IsValid() && h.IsValid();
+  }
+
   void Normalize();
 
   void Intersect(const FX_RECT& src);