FakePrefs implementation.

This patch implements a PrefsInterface subclass suitable for testing
that performs some checkings on the input/output. This class is
intended to be used in testing where currently the real Prefs class
is used on a temporary directory.

BUG=chromium:356906
TEST=Included in unit test build. Not used in any test.

Change-Id: I49ba4a687d4fd8fcafd27ad6cfbb1a61b94c1dd0
Reviewed-on: https://chromium-review.googlesource.com/195240
Tested-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Deymo <deymo@chromium.org>
diff --git a/SConstruct b/SConstruct
index 98f47a9..720c281 100644
--- a/SConstruct
+++ b/SConstruct
@@ -272,6 +272,7 @@
                             extent_mapper_unittest.cc
                             extent_ranges_unittest.cc
                             extent_writer_unittest.cc
+                            fake_prefs.cc
                             fake_system_state.cc
                             file_writer_unittest.cc
                             filesystem_copier_action_unittest.cc