IDL compiler: inheritance for interfaces

Note that this does *not* require parsing the parent interface:
we just need the *name* of the parent, which is in the IDL file itself.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/109873002

git-svn-id: svn://svn.chromium.org/blink/trunk@163396 bbb929c8-8fbe-4397-9dbb-9b2b20218538