Bug 1366511: Part 2 - Allow autoconverting Err(nsresult) to nsresult. r=ehsan,nbp

This allows MOZ_TRY and MOZ_TRY_VAR to be transparently used in XPCOM methods
when compatible Result types are used.

Also removes a compatibility macro in SimpleChannel.cpp, and an identical
specialization in AddonManagerStartup, which are no longer necessary after
this change.

MozReview-Commit-ID: 94iNrPDJEnN

toolkit/components/extensions/WebExtensionPolicy.cpp

@@ -8,6 +8,7 @@
 #include "mozilla/extensions/WebExtensionPolicy.h"
 
 #include "mozilla/AddonManagerWebAPI.h"
+#include "mozilla/ResultExtensions.h"
 #include "nsEscape.h"
 #include "nsISubstitutingProtocolHandler.h"
 #include "nsNetUtil.h"