Bug 1776074 part 4: Use references instead of pointers for nsPrintObject::Init's Document and nsDocShell args (and in callers). r=emilio

This patch doesn't change behavior. It just changes to c++ reference types and
code-comments to indicate where we know that pointers have been null-checked
(and removes some null-checks that now become trivially-unnecessary).

I've added code-comments to justify why we know these args are non-null.
Generally, `nsPrintObject::Init`'s args are null-checked by the caller, except
in one case (in `nsPrintJob::DoCommonPrint`) where the Document* pointer in
question was _not_ directly null-checked by the caller. But fortunately, it is
null-checked earlier, higher up in the call-stack.  So, this patch simply
propagates the C++ reference type-conversion up to that point for additional
clarity.

Differential Revision: https://phabricator.services.mozilla.com/D150078

layout/base/nsDocumentViewer.cpp

@@ -2934,7 +2934,9 @@ nsDocumentViewer::Print(nsIPrintSettings* aPrintSettings,
   }
 
   mPrintJob = printJob;
-  rv = printJob->Print(mDocument, aPrintSettings, aRemotePrintJob,
+
+  // Note: mDocument was null-checked earlier in this function.
+  rv = printJob->Print(*mDocument, aPrintSettings, aRemotePrintJob,
                        aWebProgressListener);
   if (NS_WARN_IF(NS_FAILED(rv))) {
     OnDonePrinting();
@@ -2979,7 +2981,8 @@ nsDocumentViewer::PrintPreview(nsIPrintSettings* aPrintSettings,
   }
   mPrintJob = printJob;
 
-  rv = printJob->PrintPreview(doc, aPrintSettings, aWebProgressListener,
+  // Note: doc is known-non-null via NS_ENSURE_STATE null-check above.
+  rv = printJob->PrintPreview(*doc, aPrintSettings, aWebProgressListener,
                               std::move(aCallback));
   if (NS_WARN_IF(NS_FAILED(rv))) {
     OnDonePrinting();