IDL compiler: sync Python to r165262

Python side to:
Add use counters for using Attrs as Nodes.
https://codereview.chromium.org/127723002

There are other cases (for interface Node itself), which I'll handle
when finalizing Node.idl.

BUG=239771
TBR=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/138033017

git-svn-id: svn://svn.chromium.org/blink/trunk@165266 bbb929c8-8fbe-4397-9dbb-9b2b20218538