Fix overflow/underflow in gfx geometry once and for all

ui/gfx/geometry/safe_integer_conversions.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_GFX_GEOMETRY_SAFE_INTEGER_CONVERSIONS_H_
 #define UI_GFX_GEOMETRY_SAFE_INTEGER_CONVERSIONS_H_
 
 #include <cmath>
 #include <limits>
 
@@ skipping 39 matching lines @@
 inline bool IsExpressibleAsInt(float value) {
   if (value != value)
     return false; // no int NaN.
   if (value > std::numeric_limits<int>::max())
     return false;
   if (value < std::numeric_limits<int>::min())
     return false;
   return true;
 }
 
+// Returns true iff a+b would overflow max int.
+constexpr bool AddWouldOverflow(int a, int b) {
+  return a > 0 && b > std::numeric_limits<int>::max() - a;
+}
+
+// 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_