Bug 1607984 - P12-4. Start parent load via DocumentChannel. r=mayhemer,nika,mattwoodrow,necko-reviewers

Add ParentProcessDocumentChannel object. This object is a DocumentChannel that will start a channel load from the parent process via a DocumentChannel.

The aim of this task is two-fold.
1- Be consistent on how we handle redirects before continuing the load on the final channel.
2- Prepare to initiate a process switch when needed without having to go via an intermediary content process, saving a process switch. This task will be done in a follow-up task.

The behaviour of the ParentProcessDocumentChannel is similar in logic to the DocumentChannelChild/DocumentChannelParent pair. The ParentProcessDocumentChannel sets up a DocumentLoadListener, have it handle the redirects and upon completion continue the load on the final channel.

Differential Revision: https://phabricator.services.mozilla.com/D70009

docshell/base/nsDocShell.cpp

@@ -5712,7 +5712,8 @@ void nsDocShell::OnRedirectStateChange(nsIChannel* aOldChannel,
   // of redirects handled in the parent process.
   // Query the full redirect chain directly, so that we can add history
   // entries for them.
-  if (RefPtr<DocumentChannel> docChannel = do_QueryObject(aOldChannel)) {
+  RefPtr<DocumentChannel> docChannel = do_QueryObject(aOldChannel);
+  if (docChannel) {
     nsCOMPtr<nsIURI> previousURI;
     uint32_t previousFlags = 0;
     docChannel->GetLastVisit(getter_AddRefs(previousURI), &previousFlags);
@@ -5760,7 +5761,7 @@ void nsDocShell::OnRedirectStateChange(nsIChannel* aOldChannel,
   // check if the new load should go through the application cache.
   nsCOMPtr<nsIApplicationCacheChannel> appCacheChannel =
       do_QueryInterface(aNewChannel);
-  if (appCacheChannel) {
+  if (appCacheChannel && !docChannel) {
     if (GeckoProcessType_Default != XRE_GetProcessType()) {
       // Permission will be checked in the parent process.
       appCacheChannel->SetChooseApplicationCache(true);