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Fixes regression introduced in CL:1068184

Also added a browsertest to verify behaviour on user kerberos files
changed D-Bus signal

BUG=chromium:845829
TEST=ExistingUserControllerActiveDirectoryTest.UserKerberosFilesChangedSignalTriggersFileUpdate

Change-Id: Ieeb800f85210c094f34c93a71b0e7502ceae3114
Reviewed-on: https://chromium-review.googlesource.com/1089060Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Roman Sorokin <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567195}