Teach missing_from_gn about gn 'inputs'

Turns out gn has 3 types of sources:
inputs (non-source files)
sources (source files)
public (source files for APIs)

I've now taught missing_from_gn about inputs
brettw says I can ignore public for now as it's unlikely
anyone is using that.

R=brettw@chromium.org

Review URL: https://codereview.chromium.org/675283002