Don't throw onPageLoad errors when triggered from Chrome's internals

When Google Chrome's "Use a prediction service to help complete searches and URLs typed in the address bar" is selected, Chrome appears to always load an https://www.google.com/webhp?... page somewhere in the background. This triggers Zotero's onPageLoad handler, but since the page is not loaded in a tab, it does not pass a tab object.

Re https://forums.zotero.org/discussion/33837/ams-mathscinet-and-euclid-translators-not-working/#Item_9