IDL compiler: Fix dropped line from rebased refactoring

A refactoring CL accidentally stomped on (didn't include) a
recently-landed CL, so it removed a feature.
This fixes it (1 line).

Specifically:

Remove class from code_generator_v8.py
https://codereview.chromium.org/140173005/
...didn't include the "add partial interface includes" line from:

IDL compiler: Partial interfaces (basic)
https://codereview.chromium.org/137793006/

BUG=239771
TBR=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/142893002

git-svn-id: svn://svn.chromium.org/blink/trunk@165375 bbb929c8-8fbe-4397-9dbb-9b2b20218538