[turbolizer] Output correct JSON when source contains a backslash.

src/ostreams.cc

Index: src/ostreams.cc
diff --git a/src/ostreams.cc b/src/ostreams.cc
index 45f41bb012089fd52a8beb14d4e6e4ad37cabb9f..a0a548b6070dafbc6f45f8a4c970bef1a4ba5220 100644
--- a/src/ostreams.cc
+++ b/src/ostreams.cc
@@ -61,6 +61,14 @@ std::ostream& PrintUC16(std::ostream& os, uint16_t c, bool (*pred)(uint16_t)) {
   return os << buf;
 }
 
+std::ostream& PrintUC16ForJSON(std::ostream& os, uint16_t c,
+                               bool (*pred)(uint16_t)) {
+  // JSON does not allow \x99; must use \u0099.
+  char buf[10];
+  const char* format = pred(c) ? "%c" : "\\u%04x";
+  snprintf(buf, sizeof(buf), format, c);
+  return os << buf;
+}
 
 std::ostream& PrintUC32(std::ostream& os, int32_t c, bool (*pred)(uint16_t)) {
   if (c <= String::kMaxUtf16CodeUnit) {
@@ -84,7 +92,7 @@ std::ostream& operator<<(std::ostream& os, const AsEscapedUC16ForJSON& c) {
   if (c.value == '\r') return os << "\\r";
   if (c.value == '\t') return os << "\\t";
   if (c.value == '\"') return os << "\\\"";
-  return PrintUC16(os, c.value, IsOK);
+  return PrintUC16ForJSON(os, c.value, IsOK);
 }