Add cipd_cache to .gitignore

Change-Id: Id4c60445a0e394abf2386339801b83b6e1533020
Reviewed-on: https://chromium-review.googlesource.com/1079696Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Paul Miller <paulmiller@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565712}

.gitignore

@@ -158,6 +158,7 @@ vs-chromium-project.txt
 /chrome_elf/chrome_elf_resources.xml
 /chromecast/internal
 /chromeos/assistant/internal
+/cipd_cache/
 /clank
 /cloud_print/cloud_print_version_resources.xml
 /components/chrome_settings_proto_generated_compile.xml