Bug 1558349. Stop calling SetIsPrintPreview in nsDocumentViewer::PrepareToStartLoad. r=bobowen

This code predates our use of a static clone to print.  Nowadays we never
navigate in the (dedicated) print preview tab.

Differential Revision: https://phabricator.services.mozilla.com/D34433

--HG--
extra : rebase_source : f4b8c03e0dde5b96e2e848892623e6f88a365095

layout/base/nsDocumentViewer.cpp

@@ -536,6 +536,9 @@ already_AddRefed<nsIContentViewer> NS_NewContentViewer() {
 }
 
 void nsDocumentViewer::PrepareToStartLoad() {
+  MOZ_DIAGNOSTIC_ASSERT(!GetIsPrintPreview(),
+                        "Print preview tab should never navigate");
+
   mStopped = false;
   mLoaded = false;
   mAttachedToParent = false;
@@ -550,9 +553,6 @@ void nsDocumentViewer::PrepareToStartLoad() {
   if (mPrintJob) {
     mPrintJob->Destroy();
     mPrintJob = nullptr;
-#  ifdef NS_PRINT_PREVIEW
-    SetIsPrintPreview(false);
-#  endif
   }
 
 #endif  // NS_PRINTING