[JSON] implement indentation in the BasicJsonStringifier and expose via API.

src/string-builder.h

Index: src/string-builder.h
diff --git a/src/string-builder.h b/src/string-builder.h
index 98bd82b97a011712e14c8365e1c007231378c211..b92bb96e5e02f3c59a0b00faacce0873e1d9b5e2 100644
--- a/src/string-builder.h
+++ b/src/string-builder.h
@@ -293,6 +293,14 @@ class IncrementalStringBuilder {
     }
   }
 
+  INLINE(void AppendCString(const uc16* s)) {
+    if (encoding_ == String::ONE_BYTE_ENCODING) {
+      while (*s != '\0') Append<uc16, uint8_t>(*(s++));
+    } else {
+      while (*s != '\0') Append<uc16, uc16>(*(s++));
+    }
+  }
+
   INLINE(bool CurrentPartCanFit(int length)) {
     return part_length_ - current_index_ > length;
   }