Skip non-critical updates during OOBE update check

If the system has not yet completed OOBE, and Omaha returns a response
with an empty deadline field, avoid initiating a background update
because doing so may violate an enterprise policy that gets pushed
down post-enrollment.

Bug: None
BUG=chromium:587101
TEST=`FEATURES=test emerge-link chromeos-base/update_engine`
TEST=powerwash, install lsb-release file from an old stable build,
     repeat OOBE, verify that download is skipped via logs, then verify
     that update proceeds after logging into a standard gmail account

Change-Id: I6e6fae02c609b2763abc6e9960bf6d87dd3534e2
diff --git a/common/error_code.h b/common/error_code.h
index 32155f2..e08ec46 100644
--- a/common/error_code.h
+++ b/common/error_code.h
@@ -72,6 +72,7 @@
   kOmahaRequestXMLHasEntityDecl = 46,
   kFilesystemVerifierError = 47,
   kUserCanceled = 48,
+  kNonCriticalUpdateInOOBE = 49,
 
   // VERY IMPORTANT! When adding new error codes:
   //