IDL compiler: [ReflectOnly]

Python side to:
Support IDL attributes limited to only known values.
https://codereview.chromium.org/124763002/

BUG=239771
R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/145773004

git-svn-id: svn://svn.chromium.org/blink/trunk@166456 bbb929c8-8fbe-4397-9dbb-9b2b20218538