Bug 1449294 - Capture thumbnails with tracking protection always enabled. r=adw

MozReview-Commit-ID: Lt389lmJASW

--HG--
extra : rebase_source : f0af8f94e3b1897d929841d192f5a52307d1f4e0

toolkit/components/thumbnails/content/backgroundPageThumbsContent.js

@@ -46,6 +46,7 @@ const backgroundPageThumbsContent = {
                        Ci.nsIWebNavigation.LOAD_FLAGS_BYPASS_HISTORY;
     docShell.defaultLoadFlags = defaultFlags;
     docShell.sandboxFlags |= SANDBOXED_AUXILIARY_NAVIGATION;
+    docShell.useTrackingProtection = true;
 
     addMessageListener("BackgroundPageThumbs:capture",
                        this._onCapture.bind(this));