IDL compiler: [ImplementedAs] in [SetWrapperReferenceTo]

* use cpp_type instead of hard-coded raw pointer
* also fix test cases so TestObject/TestObjectPython has *no* extended
  attributes on the interface (those go on TestInterface)

BUG=239771
TBR=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/176873002

git-svn-id: svn://svn.chromium.org/blink/trunk@167655 bbb929c8-8fbe-4397-9dbb-9b2b20218538