update_engine: Move scattering wait period store/load to PayloadState.

This is needed for decoupling the inference/use of this value from its
storing/loading, as we shift the former into the Update Manager.

BUG=chromium:384087
TEST=Unit tests.

Change-Id: I4b278dc817b6f148d5638122f934e9d1e280bfae
Reviewed-on: https://chromium-review.googlesource.com/221250
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
diff --git a/mock_payload_state.h b/mock_payload_state.h
index bef2997..c317543 100644
--- a/mock_payload_state.h
+++ b/mock_payload_state.h
@@ -36,6 +36,7 @@
   MOCK_METHOD0(P2PNewAttempt, void());
   MOCK_METHOD0(P2PAttemptAllowed, bool());
   MOCK_METHOD1(SetUsingP2PForDownloading, void(bool value));
+  MOCK_METHOD1(SetScatteringWaitPeriod, void(base::TimeDelta));
 
   // Getters.
   MOCK_METHOD0(GetResponseSignature, std::string());
@@ -55,6 +56,7 @@
   MOCK_METHOD0(GetP2PNumAttempts, int());
   MOCK_METHOD0(GetP2PFirstAttemptTimestamp, base::Time());
   MOCK_METHOD0(GetUsingP2PForDownloading, bool());
+  MOCK_METHOD0(GetScatteringWaitPeriod, base::TimeDelta());
 };
 
 }  // namespace chromeos_update_engine