IDL compiler: [Constructor] [Default=Undefined|NullString] optional arguments

Test-only change, as already supported by methods.

BUG=239771
NOTRY=true
TBR=haraken

Review URL: https://codereview.chromium.org/110073004

git-svn-id: svn://svn.chromium.org/blink/trunk@163838 bbb929c8-8fbe-4397-9dbb-9b2b20218538