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

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

R=dpranke@chromium.org, phajdan.jr@chromium.org

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

gclient.py

@@ -1170,6 +1170,7 @@ class GClient(Dependency):
     "linux2": "unix",
     "linux3": "unix",
     "android": "android",
+    "ios": "ios",
   }
 
   DEFAULT_CLIENT_FILE_TEXT = ("""\