IDL compiler: sync Python to r166680

Python side to:
Rename [GarbageCollected] to [WillBeGarbageCollected]
https://codereview.chromium.org/140663011

BUG=239771
R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/134683007

git-svn-id: svn://svn.chromium.org/blink/trunk@166810 bbb929c8-8fbe-4397-9dbb-9b2b20218538