Add chromium/_bad_scm to .gitignore

When Chromium decides to move conflicting dependencies
to the _bad_scm directory it is possible to end up with
problems when that directory gets added to the local index.
Ignoring it should avoid such problems.

R=hbos@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/45969004

Cr-Commit-Position: refs/heads/master@{#8988}
diff --git a/.gitignore b/.gitignore
index 3361016..467e0e7 100644
--- a/.gitignore
+++ b/.gitignore
@@ -41,6 +41,7 @@
 /chromium/.gclient.tmp
 /chromium/.gclient.tmp_entries
 /chromium/.last_sync_chromium
+/chromium/_bad_scm
 /chromium/src
 /google_apis
 /gyp-mac-tool