AU: Start checkpointing update progress.

Checkpoint the manifest metadata size and the update
check response hash in the preference store. Also checkpoint
the next operation and data offset.
Add methods for getting/setting hash context.

BUG=7390,7394
TEST=unit tests

Change-Id: I25291bf598ac9b0f1033e11cfe59df45b1f6eeab

Review URL: http://codereview.chromium.org/3521016
diff --git a/update_attempter.cc b/update_attempter.cc
index a9c9261..2db91a6 100644
--- a/update_attempter.cc
+++ b/update_attempter.cc
@@ -141,7 +141,7 @@
                              NULL,
                              new LibcurlHttpFetcher));
   shared_ptr<OmahaResponseHandlerAction> response_handler_action(
-      new OmahaResponseHandlerAction);
+      new OmahaResponseHandlerAction(prefs_));
   shared_ptr<FilesystemCopierAction> filesystem_copier_action(
       new FilesystemCopierAction(false));
   shared_ptr<FilesystemCopierAction> kernel_filesystem_copier_action(
@@ -153,7 +153,7 @@
                                  OmahaEvent::kTypeUpdateDownloadStarted),
                              new LibcurlHttpFetcher));
   shared_ptr<DownloadAction> download_action(
-      new DownloadAction(new LibcurlHttpFetcher));
+      new DownloadAction(prefs_, new LibcurlHttpFetcher));
   shared_ptr<OmahaRequestAction> download_finished_action(
       new OmahaRequestAction(prefs_,
                              omaha_request_params_,