Let jumbo ignore known strange files in sources lists

Many targets end up with stray files in their sources list,
files which trigger no toolchain. Jumbo used to assert that this
was bad but since other tools handle it silently, jumbo
should also be a bit more forgiving.

Bug: 787836
Change-Id: Ib70c3c5a053621de656af15fb4fcc413673db93c
Reviewed-on: https://chromium-review.googlesource.com/785811
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#518979}
1 file changed