Bug 1614524 - Part 3: Add a fission.remoteObjectEmbed pref, r=mattwoodrow

With these changes, there are still a number of test failures when remote
object/embed is enabled, due to svg sizing and resource timing tests. In order
to land these changes before those features are fixed, it will be placed behind
a pref.

Differential Revision: https://phabricator.services.mozilla.com/D86582

dom/base/test/test_embed_xorigin_document.html

@@ -15,6 +15,11 @@ const testPath = window.location.href.replace("http://mochi.test:8888", "");
 const testDir = testPath.substring(0, testPath.lastIndexOf('/') + 1);
 
 add_task(async function() {
+  // FIXME: Remove when bug 1658342 is fixed
+  await SpecialPowers.pushPrefEnv({
+    set: [["fission.remoteObjectEmbed", true]],
+  });
+
   info("Loading image in embed");
   let embed = document.createElement("embed");
   document.body.appendChild(embed);

modules/libpref/init/StaticPrefList.yaml

@@ -3546,6 +3546,15 @@
   value: false
   mirror: always
 
+# If true, allow process-switching documents loaded by <object> and <embed>
+# elements into a remote process.
+# NOTE: This pref has no impact outside of windows with the
+# `useRemoteSubframes` flag set.
+- name: fission.remoteObjectEmbed
+  type: bool
+  value: false
+  mirror: always
+
 #---------------------------------------------------------------------------
 # Prefs starting with "font."
 #---------------------------------------------------------------------------

netwerk/ipc/DocumentLoadListener.cpp

@@ -1654,6 +1654,11 @@ bool DocumentLoadListener::MaybeTriggerProcessSwitch(
     return false;
   }
 
+  if (!StaticPrefs::fission_remoteObjectEmbed()) {
+    LOG(("Process Switch Abort: remote <object>/<embed> disabled"));
+    return false;
+  }
+
   mObjectUpgradeHandler->UpgradeObjectLoad()->Then(
       GetMainThreadSerialEventTarget(), __func__,
       [self = RefPtr{this}, remoteType, replaceBrowsingContext, specificGroupId,