chrome.webNavigation.onCompleted fired before completion?

I'm trying to build a browser for extension for myself. The idea is that when I click the icon of the plugin, it opens a page. I then want to execute some code after that new page has finished loading but somehow it doesn't work.

var result;

chrome.browserAction.onClicked.addListener(function() {
    chrome.history.search(
        { text: "", maxResults: 100}, //object
        function(results) { //callback
            for(var item in results) {
                var currItem = results[item];
                if (currItem.url.indexOf("some_domain") > -1) {
                    result = results[item];
                    break;
                }
            }

            //Go to website
            chrome.tabs.create({
                'url': result.url
            }, function(tab) {
                new_tabId = tab.id;
            });
        }
    );
});

Now here comes the part that fails:

chrome.webNavigation.onCompleted.addListener(function(details) {
   // if (check for correct URL here) {
        var videos = document.getElementsByTagName("video");
        var video = videos[0];
        alert(videos.length); <--- always Zero! Why??
        video.load();
        video.play();

        video.addEventListener("ended", function() { ... });
   // }
});

They are both in the same background script and I do not have a content script. The permissions in the manifest are "tabs", "history", "webNavigation"

When I check with the developer console and do: document.getElementsByTagName("video").length I do get the correct number.

As implied by wOxxOm, what will prevent your code from working is that you are attempting to access the DOM from a background script. Specifically, the code:

var videos = document.getElementsByTagName("video");
var video = videos[0];
alert(videos.length); <--- always Zero! Why??
video.load();
video.play();

video.addEventListener("ended", function() { ... });

will not function in a background script. If you want to do this you will need to load/execute a content script.