IDL compiler: readonly interface attributes on Node interface

This requires a special case for is_keep_attribute_alive:
check the interface itself, in addition to the type of the attribute.

Also some cleanup to the test cases.

BUG=239771
R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/131053003

git-svn-id: svn://svn.chromium.org/blink/trunk@164764 bbb929c8-8fbe-4397-9dbb-9b2b20218538