AU: Use NiceMock<PrefsMock to suppress uninteresting call warnings.

BUG=9292
TEST=unit tests

Change-Id: I16088d203fb195e7cd3b321f3ed9b3793e8980bf

Review URL: http://codereview.chromium.org/5141004
diff --git a/update_attempter_unittest.cc b/update_attempter_unittest.cc
index 53ffc89..89adb03 100644
--- a/update_attempter_unittest.cc
+++ b/update_attempter_unittest.cc
@@ -17,6 +17,7 @@
 using testing::DoAll;
 using testing::InSequence;
 using testing::Ne;
+using testing::NiceMock;
 using testing::Property;
 using testing::Return;
 using testing::SetArgumentPointee;
@@ -56,7 +57,7 @@
 
   UpdateAttempterUnderTest attempter_;
   ActionProcessorMock* processor_;
-  PrefsMock prefs_;
+  NiceMock<PrefsMock> prefs_;
 };
 
 TEST_F(UpdateAttempterTest, RunAsRootConstructWithUpdatedMarkerTest) {