fluid_gmail.js

window.fluid.dockBadge = '';
setTimeout(updateDockBadge, 1000);
setTimeout(updateDockBadge, 3000);
setInterval(updateDockBadge, 5000);

function updateDockBadge() {

	var navigation = document.querySelector('[role=navigation]')
	var doc = navigation.contentDocument || navigation.ownerDocument;

	if (!doc) { return; }

	var anchors = [].slice.call(doc.querySelectorAll('a'))

	var result = anchors.reduce(function(prev, curr, i) {
		var match = curr.innerText.match(/\s*Inbox\s*\((\d+)\)[^\d]*/)
		if (match) return match;
		else return prev;
	}, null);

	if (result && result[1]) {
		window.fluid.dockBadge = result[1];
	}
}

Be aware that if you are not in english you have to change line 16
ex.: in french it will be ->
16. var match = curr.innerText.match(/\s_Boîte de réception\s_((\d+))[^\d]*/)

If you are using Priority Inbox, which reports in the format ($countPriorityUnread : $countOtherUnread), you want the badge to only report the unread priority emails, and you also don't want the badge to report "0", then change line 16:

var match = curr.innerText.match(/\s*Inbox\s*\(([1-9][0-9]*) : \d+\)*/)

Just in case anybody looks at this: loop at line 15 is really inefficient and something like this will eat much less CPU.