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This is the last step in factoring compute_interfaces_info:
EventInterfaces.in is a simple auxiliary file that's largely unrelated to
bindings.
Splitting it out means compute_interfaces_info now purely computes interfaces
info!
This means we can reuse it without needing dummy files (e.g., in r-b-t and in
gyp), and also simplifies its code.

We can actually simplify both scripts (and r-b-t) further,
but this is a conservative CL, just splitting it out.

BUG=341748
R=haraken

Review URL: https://codereview.chromium.org/185303008

git-svn-id: svn://svn.chromium.org/blink/trunk@168499 bbb929c8-8fbe-4397-9dbb-9b2b20218538