Add keyboard shortcut for scraping page in Chrome.

Matches https://github.com/zotero/zotero/commit/dc8998c5bc1e582173420efae7647c3add82d62c
Note that on Mac "Ctrl" is treated as "Command"

I can't find decent documentation on how to do this in Safari (I also don't have as much incentive, since I don't use it)