IDL compiler: Remove argument [TreatReturnedNullStringAs] test case

Test-only change:
[TreatReturnedNullStringAs] is only used on methods and attributes
in actual code; for parameters the closest extended attribute (which
is actually used) is [TreatNullAs=NullString],
which already has tests (immediately below).

Docs:
https://sites.google.com/a/chromium.org/dev/blink/webidl/blink-idl-extended-attributes#TOC-TreatReturnedNullStringAs-m-a-
https://sites.google.com/a/chromium.org/dev/blink/webidl/blink-idl-extended-attributes#TOC-TreatNullAs-a-p-TreatUndefinedAs-a-p-

BUG=239771
NOTRY=true
TBR=haraken

Review URL: https://codereview.chromium.org/55243007

git-svn-id: svn://svn.chromium.org/blink/trunk@161143 bbb929c8-8fbe-4397-9dbb-9b2b20218538
IDL compiler: [TreatNullAs], [TreatUndefinedAs] for arguments

Also minor refactoring to simplify string mode handling.

BUG=239771
NOTRY=true

Review URL: https://codereview.chromium.org/55663002

git-svn-id: svn://svn.chromium.org/blink/trunk@161144 bbb929c8-8fbe-4397-9dbb-9b2b20218538