Bug 1598601 - Enable DocumentChannel in the 2nd part of browser_httpToFileHistory.js r=jya

DocumentChannel is enabled by default in nightly, but not beta.  We need to
enable it explicitly in the 2nd part of this test.

Differential Revision: https://phabricator.services.mozilla.com/D54451

--HG--
extra : moz-landing-system : lando

toolkit/components/remotebrowserutils/tests/browser/browser_httpToFileHistory.js

@@ -4,6 +4,7 @@ const { E10SUtils } = ChromeUtils.import(
 
 const RESPONSE_PROCESS_SELECTION_PREF =
   "browser.tabs.remote.useHTTPResponseProcessSelection";
+const DOCUMENT_CHANNEL_PREF = "browser.tabs.documentchannel";
 const FISSION_PREF = "fission.autostart";
 const HISTORY = [
   { url: httpURL("dummy_page.html") },
@@ -129,7 +130,10 @@ add_task(async function prefDisabled() {
 
 add_task(async function prefEnabled() {
   await SpecialPowers.pushPrefEnv({
-    set: [[RESPONSE_PROCESS_SELECTION_PREF, true]],
+    set: [
+      [RESPONSE_PROCESS_SELECTION_PREF, true],
+      [DOCUMENT_CHANNEL_PREF, true],
+    ],
   });
   await runTest();
 });