Bug 1900392 - Isolate x-moz-ews URI scheme to parent process. r=nika,jesup,smaug

Thunderbird architecture does not currently allow for redirecting channels to
child processes, so new schemes need to be isolated to the parent process.
`x-moz-ews` was selected to clearly designate that the scheme is private and
restricted to Mozilla/MZLA code. It doesn't follow the RFC 7595 recommendation
to use a registered domain as a namespace, but this was felt to be unwieldy.

Differential Revision: https://phabricator.services.mozilla.com/D212439

docshell/base/nsDocShell.cpp

@@ -9576,7 +9576,7 @@ bool nsDocShell::CanLoadInParentProcess(nsIURI* aURI) {
 #ifdef MOZ_THUNDERBIRD
   if (uri->SchemeIs("imap") || uri->SchemeIs("mailbox") ||
       uri->SchemeIs("news") || uri->SchemeIs("nntp") ||
-      uri->SchemeIs("snews")) {
+      uri->SchemeIs("snews") || uri->SchemeIs("x-moz-ews")) {
     return true;
   }
 #endif