AppShell: pass command-line files to app as launch data

An app that registers file handlers can be passed filenames on the
command line to use those files.

BUG=679870

Original Review-Url: https://codereview.chromium.org/2824553002

Review-Url: https://codereview.chromium.org/2866073002
Cr-Commit-Position: refs/heads/master@{#470810}
5 files changed