Scatter downloads to reduce bandwidth spikes.

Support in update_engine to honor the enterprise policy to scatter the
downloading of ChromeOS automatic updates so that we reduce bandwidth
spikes caused due to simultaneous downloads of updates by a large number
of enterprise devices.

This has no effect on consumer devices.

BUG=chromeos-29615: Implement scattering of downloads in UpdateEngine
TEST=Manually tested all scenarios, Unit tests added for all new code.
CQ-DEPEND=I1f56b5516970d5988eebb2cf8f93f6905823801d
Change-Id: I4a8f4974467a064d723ab13cbd78b1ca3ceff420
Reviewed-on: https://gerrit.chromium.org/gerrit/21574
Commit-Ready: Jay Srinivasan <jaysri@chromium.org>
Reviewed-by: Jay Srinivasan <jaysri@chromium.org>
Tested-by: Jay Srinivasan <jaysri@chromium.org>
diff --git a/prefs_mock.h b/prefs_mock.h
index cf030f0..963b981 100644
--- a/prefs_mock.h
+++ b/prefs_mock.h
@@ -1,4 +1,4 @@
-// Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
+// Copyright (c) 2012 The Chromium OS Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
@@ -17,6 +17,9 @@
                                const std::string& value));
   MOCK_METHOD2(GetInt64, bool(const std::string& key, int64_t* value));
   MOCK_METHOD2(SetInt64, bool(const std::string& key, const int64_t value));
+
+  MOCK_METHOD1(Exists, bool(const std::string& key));
+  MOCK_METHOD1(Delete, bool(const std::string& key));
 };
 
 }  // namespace chromeos_update_engine