Bug 241356

I got the following stacktrace while building WebKitGTK:

```
Traceback (most recent call last):
  File "/home/dpino/playwright/browser_patches/webkit/checkout/Source/WebKit/Scripts/generate-message-receiver.py", line 92, in <module>
	sys.exit(main(sys.argv))
  File "/home/dpino/playwright/browser_patches/webkit/checkout/Source/WebKit/Scripts/generate-message-receiver.py", line 52, in main
	receiver = webkit.parser.parse(source_file)
  File "/home/dpino/playwright/browser_patches/webkit/checkout/Source/WebKit/Scripts/webkit/parser.py", line 52, in parse
	for line in file:
  File "/usr/lib/python3.6/encodings/ascii.py", line 26, in decode
	return codecs.ascii_decode(input, self.errors)[0]
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc5 in position 1776: ordinal not in range(128)
```

Debugging the list of parsed files, it seems an extra non-ascii character in "Source/WebKit/Shared/IPCTester.messages.in" caused the parsing error.