Fix IDL dependency generation non-determinacy

There's a small indeterminacy in compute_dependencies.py,
due to dictionary key order being non-deterministic.
This fixes it, and also simplifies the code.
(E.g., all the extended attributes take a required argument,
 so we can skip a test for it, and list comprehensions are more
 idiomatic.)

Only generated code change is order of extended attributes
(and hence order of attributes in Window.idl).

Review URL: https://codereview.chromium.org/47923007

git-svn-id: svn://svn.chromium.org/blink/trunk@160824 bbb929c8-8fbe-4397-9dbb-9b2b20218538