tools/gn: fix escaping of backslashes

tools/gn/value_unittest.cc

Index: tools/gn/value_unittest.cc
diff --git a/tools/gn/value_unittest.cc b/tools/gn/value_unittest.cc
index 9cbdaa2dda3699cede165227ec355163c4bcd85c..f934e6601812ec48e3caf8b6c8592a83bd3de12f 100644
--- a/tools/gn/value_unittest.cc
+++ b/tools/gn/value_unittest.cc
@@ -11,6 +11,10 @@ TEST(Value, ToString) {
   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));
 
@@ -35,4 +39,3 @@ TEST(Value, ToString) {
   scope->SetValue("b", Value(nullptr, "hello, world"), nullptr);
   EXPECT_EQ("{\n  a = 42\n  b = \"hello, world\"\n}", scopeval.ToString(false));
 }
-