Include "fuchsia" as a known "deps_os" value

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

R=machenbach@chromium.org, tandrii@chromium.org, wez@chromium.org

Bug: 848676
Change-Id: I782184b615968140d0e9a0146169fa12a9ebe0c5
Reviewed-on: https://chromium-review.googlesource.com/1084211
Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Sergiy Byelozyorov <sergiyb@chromium.org>

gclient.py

@@ -1268,6 +1268,7 @@ class GClient(GitDependency):
     "linux3": "unix",
     "android": "android",
     "ios": "ios",
+    "fuchsia": "fuchsia",
   }
 
   DEFAULT_CLIENT_FILE_TEXT = ("""\