GN: Move mock_pref_change_callback.h to the correct section.

GN: Move mock_pref_change_callback.h to the correct section.

This resolves the following 'gn check' error:

  ERROR at //base/prefs/mock_pref_change_callback.cc:5:11: Include not allowed.
  #include "base/prefs/mock_pref_change_callback.h"
            ^-------------------------------------
  It is not in any dependency of //base:prefs_test_support
  The include file is in the target(s):
    //base:base_unittests
  which should somehow be reachable from //base:prefs_test_support

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=282042

[jbroman, 2014-07-07 21:04:07]

I'm not a base/ expert, but I don't see why this shouldn't just be in
//base:prefs_test_support.

[brettw, 2014-07-08 21:17:58]

lgtm

[jbroman, 2014-07-09 13:15:34]

The CQ bit was checked by jbroman@chromium.org

[commit-bot: I haz the power, 2014-07-09 13:16:38]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jbroman@chromium.org/378653002/1

[commit-bot: I haz the power, 2014-07-09 15:15:43]

Change committed as 282042