[epiphany/pgriffis/web-extension-beastify] WebExtension: Stub out extension.getBackgroundPage()

[Patrick Griffis <pgriffis src gnome org>]

commit 689c92a9595add3251e48fc01aa5f7d056734f47
Author: Patrick Griffis <pgriffis igalia com>
Date:   Mon May 23 13:20:17 2022 -0500

    WebExtension: Stub out extension.getBackgroundPage()

 embed/web-process-extension/resources/js/webextensions.js | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/embed/web-process-extension/resources/js/webextensions.js 
b/embed/web-process-extension/resources/js/webextensions.js
index f0edce2ce..7fafff903 100644
--- a/embed/web-process-extension/resources/js/webextensions.js
+++ b/embed/web-process-extension/resources/js/webextensions.js
@@ -86,3 +86,6 @@ window.browser.browserAction = {
 window.browser.windows = {
   WINDOW_ID_CURRENT: -2, /* Matches Firefox, used in tabs.c. */
 };
+
+/* Firefox returns null in private mode. So extensions sometimes handle this. */
+window.browser.extension.getBackgroundPage = function () { return null; };