Add dummy DEPS file for patched YASM.

This will allow clients that use the subtree mirror to specify a
revision for patched YASM without needing nested DEPS.

Also cleans up the gitignore for the subtree mirror so gclient
revert will work properly.

Bug: 1017514
Change-Id: I7a3a51d1ab79437dd315a246b833211d0ef08df3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1877810
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#709539}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 412bd0e3aff5a1b93561db69a44b4a55ddc65dc3