update_engine: Reject XML with internal entity declarations.

This helps avoid resource exhaustion problems.

BUG=chromium:406546
TEST=New unit test + unit tests pass.

Change-Id: Ib54f378cf533c200631b274c0414075c2ea4ff67
Reviewed-on: https://chromium-review.googlesource.com/214291
Reviewed-by: Chris Masone <cmasone@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Commit-Queue: David Zeuthen <zeuthen@chromium.org>
Tested-by: David Zeuthen <zeuthen@chromium.org>
diff --git a/payload_state_unittest.cc b/payload_state_unittest.cc
index 17f7aea..6646e56 100644
--- a/payload_state_unittest.cc
+++ b/payload_state_unittest.cc
@@ -91,7 +91,7 @@
 class PayloadStateTest : public ::testing::Test { };
 
 TEST(PayloadStateTest, DidYouAddANewErrorCode) {
-  if (static_cast<int>(ErrorCode::kUmaReportedMax) != 46) {
+  if (static_cast<int>(ErrorCode::kUmaReportedMax) != 47) {
     LOG(ERROR) << "The following failure is intentional. If you added a new "
                << "ErrorCode enum value, make sure to add it to the "
                << "PayloadState::UpdateFailed method and then update this test "