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

test/cctest/interpreter/bytecode_expectations/RegExpLiterals.golden

 #
 # Autogenerated by generate-bytecode-expectations.
 #
 
 ---
 pool type: string
 execute: yes
 wrap: yes
 
 ---
@@ skipping 20 matching lines @@
 "
 frame size: 0
 parameter count: 1
 bytecode array length: 6
 bytecodes: [
   /*   30 E> */ B(StackCheck),
   /*   34 S> */ B(CreateRegExpLiteral), U8(0), U8(0), U8(2),
   /*   58 S> */ B(Return),
 ]
 constant pool: [
-  "(\x5cw+)\x5cs(\x5cw+)",
+  "(\u005cw+)\u005cs(\u005cw+)",
 ]
 handlers: [
 ]
 
 ---
 snippet: "
   return /ab+d/.exec('abdd');
 "
 frame size: 3
 parameter count: 1
 bytecode array length: 22
 bytecodes: [
   /*   30 E> */ B(StackCheck),
   /*   34 S> */ B(CreateRegExpLiteral), U8(0), U8(0), U8(0),
                 B(Star), R(1),
   /*   47 E> */ B(LdrNamedProperty), R(1), U8(1), U8(3), R(0),
                 B(LdaConstant), U8(2),
                 B(Star), R(2),
   /*   48 E> */ B(Call), R(0), R(1), U8(2), U8(1),
   /*   62 S> */ B(Return),
 ]
 constant pool: [
   "ab+d",
   "exec",
   "abdd",
 ]
 handlers: [
 ]