IDL compiler: Cleanup TestObject.idl and attribute getter template

This cleans up TestObject.idl, removing cases that aren't used.
This allows us to turn it on in Python: all cases pass!

* 'create' attribute (special name)
* [CallWith=ScriptState] attributes
* [CallWith=ExecutionContext] + [RaisesException=Getter] (just order)
* nullable + [RaisesException=Getter] (just order)
* methods with nullable arguments
　(property setters with nullable arguments work fine, but no normal methods)

Also clean up attribute getter in attributes.cpp,
giving some order and putting in FIXMEs for post-Perl cleanup.

BUG=239771
TBR=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/142923004

git-svn-id: svn://svn.chromium.org/blink/trunk@166679 bbb929c8-8fbe-4397-9dbb-9b2b20218538