IDL compiler: [ImplementedAs] interfaces

Test-only change, as already supported in code.

BUG=239771
NOTRY=true
TBR=haraken

Review URL: https://codereview.chromium.org/103593002

git-svn-id: svn://svn.chromium.org/blink/trunk@163143 bbb929c8-8fbe-4397-9dbb-9b2b20218538