bind-gen: Make LooseFormatter support {} and {0}

Make LooseFormatter support implicit ({}) and explicit ({0}, {1},
...) positional argument specifiers.

Bug: 839389
Change-Id: I8a820cba90a2be580e63c15d35c194c4ba31c702
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1897575
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712459}

third_party/blink/renderer/bindings/scripts/bind_gen/code_node.py

@@ -64,13 +64,25 @@ class CodeNode(object):
             self.undefined_code_symbols = []
 
     class _LooseFormatter(string.Formatter):
+        def __init__(self):
+            string.Formatter.__init__(self)
+            self._loose_formatter_indexing_count_ = 0
+
         def get_value(self, key, args, kwargs):
+            if isinstance(key, (int, long)):
+                return args[key]
+            assert isinstance(key, str)
+            if not key:
+                # Before Python 3.1, when a positional argument specifier is
+                # omitted, |format_string="{}"| produces |key=""|.
+                index = self._loose_formatter_indexing_count_
+                self._loose_formatter_indexing_count_ += 1
+                return args[index]
             if key in kwargs:
                 return kwargs[key]
             else:
                 return "{" + key + "}"
 
-    _template_formatter = _LooseFormatter()
     _gensym_seq_id = 0
 
     @classmethod
@@ -83,7 +95,7 @@ class CodeNode(object):
             "${template_var} 42"
         without raising an exception that |template_var| is unbound.
         """
-        return cls._template_formatter.format(format_string, *args, **kwargs)
+        return cls._LooseFormatter().format(format_string, *args, **kwargs)
 
     @classmethod
     def gensym(cls):