Fix overflow/underflow in gfx geometry once and for all

ui/gfx/geometry/vector2d.cc

 // 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.
 
 #include "ui/gfx/geometry/vector2d.h"
 
 #include <cmath>
 
 #include "base/strings/stringprintf.h"
+#include "ui/gfx/geometry/safe_integer_conversions.h"
 
 namespace gfx {
 
 bool Vector2d::IsZero() const {
   return x_ == 0 && y_ == 0;
 }
 
 void Vector2d::Add(const Vector2d& other) {
-  x_ += other.x_;
-  y_ += other.y_;
+  x_ = SafeAdd(other.x_, x_);
+  y_ = SafeAdd(other.y_, y_);
 }
 
 void Vector2d::Subtract(const Vector2d& other) {
-  x_ -= other.x_;
-  y_ -= other.y_;
+  x_ = SafeSubtract(x_, other.x_);
+  y_ = SafeSubtract(y_, other.y_);
 }
 
 int64_t Vector2d::LengthSquared() const {
   return static_cast<int64_t>(x_) * x_ + static_cast<int64_t>(y_) * y_;
 }
 
 float Vector2d::Length() const {
   return static_cast<float>(std::sqrt(static_cast<double>(LengthSquared())));
 }
 
 std::string Vector2d::ToString() const {
   return base::StringPrintf("[%d %d]", x_, y_);
 }
 
 }  // namespace gfx