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 <[email protected]>
Reviewed-by: Alex Deymo <[email protected]>
Commit-Queue: David Zeuthen <[email protected]>
Tested-by: David Zeuthen <[email protected]>
diff --git a/error_code.h b/error_code.h
index 567534a..6a47784 100644
--- a/error_code.h
+++ b/error_code.h
@@ -57,6 +57,7 @@
   kPostinstallFirmwareRONotUpdatable = 43,
   kUnsupportedMajorPayloadVersion = 44,
   kUnsupportedMinorPayloadVersion = 45,
+  kOmahaRequestXMLHasEntityDecl = 46,
 
   // VERY IMPORTANT! When adding new error codes:
   //