AU: Fall back to a new update after a certain number of failed resume attempts.

BUG=7483
TEST=unit tests, gmerged on device and inspected preferences

Change-Id: I9d184b314c359f32a9791afd8da7eb6607f428eb

Review URL: http://codereview.chromium.org/3575024
diff --git a/prefs_interface.h b/prefs_interface.h
index 66fe8cb..81aa770 100644
--- a/prefs_interface.h
+++ b/prefs_interface.h
@@ -13,6 +13,7 @@
 extern const char kPrefsLastActivePingDay[];
 extern const char kPrefsLastRollCallPingDay[];
 extern const char kPrefsManifestMetadataSize[];
+extern const char kPrefsResumedUpdateFailures[];
 extern const char kPrefsUpdateCheckResponseHash[];
 extern const char kPrefsUpdateStateNextDataOffset[];
 extern const char kPrefsUpdateStateNextOperation[];