I want to use autoninja from //build/android/generate_gradle.py, but I
invoke it with a *long* list of targets that I do not want echo'ed

Bug: 803493
Change-Id: I98ad85b64ec7ce60b7e3df9827fb613cd57638e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1596535Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>