Fix V8 extras js2c pipeline

570fca61 introduced a regression that caused minification etc. to be
applied to V8 extras.

R=yangguo@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1125103003

Cr-Commit-Position: refs/heads/master@{#28427}

tools/js2c.py

@@ -415,7 +415,7 @@ def PrepareSources(source_files, native_type, emit_js):
     message_template_file = message_template_files[0]
 
   filters = None
-  if native_type == "EXTRA":
+  if native_type == "EXTRAS":
     filters = BuildExtraFilterChain()
   else:
     filters = BuildFilterChain(macro_file, message_template_file)