Clean up compute_interfaces_info.py

This is the compute_interfaces_info.py side of the post-split cleanup.
This finishes the build factoring.
(Still need to add more caching for speed and some more use
of Jinja for clarity, but mostly done with build.)

Two notable changes:
* We only need to store *inherited* extended attributes, not all extended
　attributes (in auxiliary variable; needed more for event interfaces, gone now).
　This simplifies the inheritance resolution, and reduces duplication.

* Always include the same keys in the info dict, even if the value is empty.
　This simplifies both setting (since no test) and use:
　instead of having "if key in dict and dict[key]..."
　we can just have "if dict[key]".
　It also helps consistency, since we were doing it 2 ways before.

BUG=341748
R=haraken

Review URL: https://codereview.chromium.org/183853024

git-svn-id: svn://svn.chromium.org/blink/trunk@168610 bbb929c8-8fbe-4397-9dbb-9b2b20218538