Revert of Revert of tools/gn: fix escaping of backslashes

tools/gn/value_unittest.cc

 // Copyright 2014 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 "testing/gtest/include/gtest/gtest.h"
 #include "tools/gn/test_with_scope.h"
 #include "tools/gn/value.h"
 
 TEST(Value, ToString) {
   Value strval(nullptr, "hi\" $me\\you\\$\\\"");
   EXPECT_EQ("hi\" $me\\you\\$\\\"", strval.ToString(false));
   EXPECT_EQ("\"hi\\\" \\$me\\you\\\\\\$\\\\\\\"\"", strval.ToString(true));
 
+  // crbug.com/470217
+  Value strval2(nullptr, "\\foo\\\\bar\\");
+  EXPECT_EQ("\"\\foo\\\\\\bar\\\\\"", strval2.ToString(true));
+
   // Void type.
   EXPECT_EQ("<void>", Value().ToString(false));
 
   // Test lists, bools, and ints.
   Value listval(nullptr, Value::LIST);
   listval.list_value().push_back(Value(nullptr, "hi\"me"));
   listval.list_value().push_back(Value(nullptr, true));
   listval.list_value().push_back(Value(nullptr, false));
   listval.list_value().push_back(Value(nullptr, static_cast<int64>(42)));
   // Printing lists always causes embedded strings to be quoted (ignoring the
   // quote flag), or else they wouldn't make much sense.
   EXPECT_EQ("[\"hi\\\"me\", true, false, 42]", listval.ToString(false));
   EXPECT_EQ("[\"hi\\\"me\", true, false, 42]", listval.ToString(true));
 
   // Scopes.
   TestWithScope setup;
   Scope* scope = new Scope(setup.scope());
   Value scopeval(nullptr, scoped_ptr<Scope>(scope));
   EXPECT_EQ("{ }", scopeval.ToString(false));
 
   scope->SetValue("a", Value(nullptr, static_cast<int64>(42)), nullptr);
   scope->SetValue("b", Value(nullptr, "hello, world"), nullptr);
   EXPECT_EQ("{\n  a = 42\n  b = \"hello, world\"\n}", scopeval.ToString(false));
 }
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