Bug 1657036, readd the line that deactivates the docshell for the thumbnailer browser when it is created. Also, move the activating line to after the load occurs as it previously was, r=Gijs

Differential Revision: https://phabricator.services.mozilla.com/D86092

toolkit/actors/BackgroundThumbnailsChild.jsm

@@ -52,8 +52,6 @@ class BackgroundThumbnailsChild extends JSWindowActorChild {
           .loadGroup.QueryInterface(Ci.nsISupportsPriority).priority =
           Ci.nsISupportsPriority.PRIORITY_LOWEST;
 
-        docShell.isActive = true;
-
         docShell.allowMedia = false;
         docShell.allowPlugins = false;
         docShell.allowContentRetargeting = false;

toolkit/components/thumbnails/BackgroundPageThumbs.jsm

@@ -411,6 +411,7 @@ const BackgroundPageThumbs = {
     });
 
     this._thumbBrowser = browser;
+    browser.docShellIsActive = false;
   },
 
   _destroyBrowser() {
@@ -619,6 +620,8 @@ Capture.prototype = {
       return;
     }
 
+    aBrowser.docShellIsActive = true;
+
     // The requested page has loaded or stopped/aborted, so capture the page
     // soon but first let it settle in case of in-page redirects
     await new Promise(resolve => {