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/prefs.cc b/prefs.cc
index bdd620b..223fc9b 100644
--- a/prefs.cc
+++ b/prefs.cc
@@ -35,7 +35,11 @@
 bool Prefs::GetString(const string& key, string* value) {
   FilePath filename;
   TEST_AND_RETURN_FALSE(GetFileNameForKey(key, &filename));
-  TEST_AND_RETURN_FALSE(file_util::ReadFileToString(filename, value));
+  if (!file_util::ReadFileToString(filename, value)) {
+    PLOG(INFO) << "Unable to read prefs from " << filename.value()
+               << ". This is likely harmless.";
+    return false;
+  }
   return true;
 }