Fix overflow/underflow in gfx geometry once and for all

ui/gfx/geometry/safe_integer_conversions.h

Index: ui/gfx/geometry/safe_integer_conversions.h
diff --git a/ui/gfx/geometry/safe_integer_conversions.h b/ui/gfx/geometry/safe_integer_conversions.h
index 5efe134f0793ba51f1ba0908e04b9c987e638073..eb3c8037fd34002357e2b85298852a491356762e 100644
--- a/ui/gfx/geometry/safe_integer_conversions.h
+++ b/ui/gfx/geometry/safe_integer_conversions.h
@@ -57,6 +57,52 @@ inline bool IsExpressibleAsInt(float value) {
   return true;
 }
 
+// Returns true iff a+b would overflow max int.
+constexpr bool AddWouldOverflow(int a, int b) {
+  // In this function, GCC tries to make optimizations that would only
+  // work if max - a wouldn't overflow but it isn't smart enough to notice that
+  // a > 0.  So cast everything to unsigned to avoid this.  As it is guaranteed
+  // that max - a and b are both already positive, the cast is a noop.
+  //
+  // This is intended to be: a > 0 && max - a < b
+  return a > 0 && b > 0 &&
+         static_cast<unsigned>(std::numeric_limits<int>::max() - a) <
+             static_cast<unsigned>(b);
+}
+
+// Returns true iff a+b would underflow min int.
+constexpr bool AddWouldUnderflow(int a, int b) {
+  return a < 0 && b < std::numeric_limits<int>::min() - a;
+}
+
+// Returns true iff a-b would overflow max int.
+constexpr bool SubtractWouldOverflow(int a, int b) {
+  return b < 0 && a > std::numeric_limits<int>::max() + b;
+}
+
+// Returns true iff a-b would underflow min int.
+constexpr bool SubtractWouldUnderflow(int a, int b) {
+  return b > 0 && a < std::numeric_limits<int>::min() + b;
+}
+
+// Safely adds a+b without integer overflow/underflow.  Exceeding these
+// bounds will clamp to the max or min int limit.
+constexpr int SafeAdd(int a, int b) {
+  return AddWouldOverflow(a, b)
+             ? std::numeric_limits<int>::max()
+             : AddWouldUnderflow(a, b) ? std::numeric_limits<int>::min()
+                                       : a + b;
+}
+
+// Safely subtracts a-b without integer overflow/underflow.  Exceeding these
+// bounds will clamp to the max or min int limit.
+constexpr int SafeSubtract(int a, int b) {
+  return SubtractWouldOverflow(a, b)
+             ? std::numeric_limits<int>::max()
+             : SubtractWouldUnderflow(a, b) ? std::numeric_limits<int>::min()
+                                            : a - b;
+}
+
 }  // namespace gfx
 
 #endif  // UI_GFX_GEOMETRY_SAFE_INTEGER_CONVERSIONS_H_