Include "ios" as a known "deps_os" value.
Include "ios" as a known "deps_os" value.

This fixes an issue where "--deps=all" syncs everything but ios.

R=6693ee08b084bb2ffa767f664c99b689395e0e4b@chromium.org, phajdan.jr@chromium.org

Change-Id: I413269c5026b2c04f0d34d9ca874057eb35e1076
Reviewed-on: https://chromium-review.googlesource.com/618802
Reviewed-by: Dirk Pranke <6693ee08b084bb2ffa767f664c99b689395e0e4b@chromium.org>
Commit-Queue: Michael Moss <9cc3612fe50cad025435ae878219b6203d6b641d@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools