AU: Push seeks in http fetching to the progress percentage.

The updater was reporting status as a fraction of bytes downloaded vs
total, ignoring any bytes that were skipped due to resuming an
upload. The result was that a user may see progress go from 0 to 60
percent, then reboot, then see progress go from 0 to 40 percent, then
finish.

This CL allows an HTTP fetcher to report to the delegate that it's
seeking to a particular offset.

BUG=8151
TEST=applied updates on the machine; unittest

Review URL: http://codereview.chromium.org/4131005

Change-Id: Ib9fe034ca3ffd17455af4cf89d5b28ec236104a0
diff --git a/download_action.cc b/download_action.cc
index b6f7f81..bb5d189 100644
--- a/download_action.cc
+++ b/download_action.cc
@@ -107,6 +107,10 @@
   }
 }
 
+void DownloadAction::SeekToOffset(off_t offset) {
+  bytes_received_ = offset;
+}
+
 void DownloadAction::ReceivedBytes(HttpFetcher *fetcher,
                                    const char* bytes,
                                    int length) {