IDL compiler: sync Python to r167772

Python side to:
Add context to generated named and indexed property operations.
https://codereview.chromium.org/163883006

TBR=haraken
BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/179553007

git-svn-id: svn://svn.chromium.org/blink/trunk@167843 bbb929c8-8fbe-4397-9dbb-9b2b20218538