Add .gitignore file for Firebase CIPD checkout to ignore untracked files

Bug: 866927
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I25d41139da731ae998ac82e1643c8b261cf7b886
Reviewed-on: https://chromium-review.googlesource.com/1149700Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Peter Lee <pkl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577881}

ios/third_party/firebase/.gitignore

+/Analytics
+/NOTICES
+/README.md