borealis: Add placeholder icons

For now these icons are only used by the installer. Going forward they
will be used as a fallback icon in the event that a borealis app doesn't
provide its own.

These PNGs were optimized with:
tools/resources/optimize-png-files.sh -o2 chrome/app/theme/borealis/

Bug: b/163431457
Change-Id: I4e95720093c137d3a5e64ddca8c9a17a905844ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377770


Commit-Queue: Nic Hollingum <hollingum@google.com>
Reviewed-by: Nancy Wang <nancylingwang@chromium.org>
Reviewed-by: Mitsuru Oshima (Slow: gardener) <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805632}