Add comment to network service's pref dependency.

NOTRY=true

Bug: none
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I569974af395e6afd27ca2ef41e358704428cd01c
Reviewed-on: https://chromium-review.googlesource.com/779247
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518265}

content/network/DEPS

 # Tight rules since this will move to services/network. See README.
 include_rules = [
   "+components/network_session_configurator",
+  # Prefs are used to create an independent file with a persisted key:value
+  # store for networking-related data (Like which servers support QUIC), rather
+  # than to store user preferences.
   "+components/prefs",
   "-content",
   "+content/common/content_export.h",