Bug 1768907 - Part 2: nsTransferable disk-leaks when private browsing mode is...

Bug 1768907 - Part 2: nsTransferable disk-leaks when private browsing mode is enabled by default. r=handyman

nsTransferable was modified to prevent disk leakings when copying data
in private browsing mode with Bug 1123480.
However, the context is nullptr when it is initialized, so it still
leaks if PBM is enabled by default.
Our solution is to check the browser.privatebrowsing.autostart in this
condition.

Differential Revision: https://phabricator.services.mozilla.com/D157800

widget/nsTransferable.cpp

@@ -32,6 +32,7 @@ Notes to self:
 #include "nsNetUtil.h"
 #include "nsILoadContext.h"
 #include "nsXULAppAPI.h"
+#include "mozilla/StaticPrefs_browser.h"
 #include "mozilla/UniquePtr.h"
 
 using namespace mozilla;
@@ -195,6 +196,10 @@ nsTransferable::Init(nsILoadContext* aContext) {
 
   if (aContext) {
     mPrivateData = aContext->UsePrivateBrowsing();
+  } else {
+    // without aContext here to provide PrivateBrowsing information, we defer to
+    // the active configured setting
+    mPrivateData = StaticPrefs::browser_privatebrowsing_autostart();
   }
 #ifdef DEBUG
   mInitialized = true;