bind-gen: Improve SymbolScopeNode._insert_symbol_definition

Re-implements SymbolScopeNode._insert_symbol_definition to
cover more general combinations of SequenceNode and
SymbolScopeNode.

Bug: 839389
Change-Id: I3a9163e2a47795e5e5035edf7021ef1d1d715432
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1948685Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721794}

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

@@ -85,7 +85,15 @@ class MakoRenderer(object):
         return text
 
     @property
-    def callers(self):
+    def callers_from_first_to_last(self):
+        """
+        Returns the callers of this renderer in the order from the first caller
+        to the last caller.
+        """
+        return iter(self._caller_stack)
+
+    @property
+    def callers_from_last_to_first(self):
         """
         Returns the callers of this renderer in the order from the last caller
         to the first caller.