IDL compiler: sync Python to r163665

Simplify invokeCallback() and support void return values for IDL callbacks
https://codereview.chromium.org/111603006/

Also cleanup test cases to reflect use of void,
and rename to clarify callback *interface*.

BUG=239771
NOTRY=true
TBR=haraken

Review URL: https://codereview.chromium.org/112383002

git-svn-id: svn://svn.chromium.org/blink/trunk@163667 bbb929c8-8fbe-4397-9dbb-9b2b20218538