Put experimental extras through the same filters as non-experimental

When introducing experimental extras I forgot to update this check. That resulted in experimental extras running through the steps normally reserved for non-extra natives: macro and constant expansion, validation, and minification. This causes problems since per the linked bug minification is buggy.

R=yangguo@chromium.org, jochen@chromium.org
BUG=v8:4064
LOG=N

Review URL: https://codereview.chromium.org/1400253002

Cr-Commit-Position: refs/heads/master@{#31270}

tools/js2c.py

@@ -417,7 +417,7 @@ def PrepareSources(source_files, native_type, emit_js):
     message_template_file = message_template_files[0]
 
   filters = None
-  if native_type == "EXTRAS":
+  if native_type in ("EXTRAS", "EXPERIMENTAL_EXTRAS"):
     filters = BuildExtraFilterChain()
   else:
     filters = BuildFilterChain(macro_file, message_template_file)