IDL compiler: sync Python to r164407

Python side to:
Remove custom signatures from V8 bindings
https://codereview.chromium.org/109473008/

BUG=239771
TBR=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/114693005

git-svn-id: svn://svn.chromium.org/blink/trunk@164488 bbb929c8-8fbe-4397-9dbb-9b2b20218538