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.
Merge pull request #17 from aurimasv/no-tab

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