IDL compiler: [GarbageCollected] on other interface

[GarbageCollected] affects not only bindings a given interface
(and descendants), but also bindings for all interfaces that *use* it:
it's external information.

Add support to compute_dependencies.py for this;
since we already inherit [GarbageCollected], it's just a
matter of making the code more generic.

BUG=239771
R=haraken

Review URL: https://codereview.chromium.org/151173004

git-svn-id: svn://svn.chromium.org/blink/trunk@166598 bbb929c8-8fbe-4397-9dbb-9b2b20218538