Fix the build with WEB_AUDIO disabled.

tools/dom/scripts/systemnative.py

Index: tools/dom/scripts/systemnative.py
diff --git a/tools/dom/scripts/systemnative.py b/tools/dom/scripts/systemnative.py
index 4bc95a5d6ef945c004304cbe42184d4a094da5d6..7786c0c81d1c77dcef7764b21e34f8ae3e339ec5 100644
--- a/tools/dom/scripts/systemnative.py
+++ b/tools/dom/scripts/systemnative.py
@@ -10,7 +10,7 @@ import emitter
 import os
 from generator import *
 from htmldartgenerator import *
-from systemhtml import js_support_checks, GetCallbackInfo
+from systemhtml import js_support_checks, GetCallbackInfo, HTML_LIBRARY_NAMES
 
 class DartiumBackend(HtmlDartGenerator):
   """Generates Dart implementation for one DOM IDL interface."""
@@ -875,7 +875,7 @@ class CPPLibraryEmitter():
   def __init__(self, emitters, cpp_sources_dir):
     self._emitters = emitters
     self._cpp_sources_dir = cpp_sources_dir
-    self._library_headers = {}
+    self._library_headers = dict((lib, []) for lib in HTML_LIBRARY_NAMES)
     self._sources_list = []
 
   def CreateHeaderEmitter(self, interface_name, library_name, is_callback=False):