IDL compiler: [StrictTypeChecking], [TreatNullAs] (test stubs only)

Setter-only, just add stub tests (assert nop for getters).
Also fixes valid values for TreatNullAs/TreatUndefinedAs:
these are generally NullString, but can take other values for
indexed setters/named setters

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/27476002

git-svn-id: svn://svn.chromium.org/blink/trunk@159745 bbb929c8-8fbe-4397-9dbb-9b2b20218538