Bare minimum Chrome extension to inject a JS file into the given page when you click on the browser action icon. The script then inserts a new div into the DOM.

background.js

// this is the background code...

// listen for our browerAction to be clicked
chrome.browserAction.onClicked.addListener(function (tab) {
	// for the current tab, inject the "inject.js" file & execute it
	chrome.tabs.executeScript(tab.ib, {
		file: 'inject.js'
	});
});

inject.js

// this is the code which will be injected into a given page...

(function() {
	
walk(document.body);

function walk(node) 
{
	// I stole this function from here:
	// http://is.gd/mwZp7E
	
	var child, next;
	
	if (node.tagName.toLowerCase() == 'input' || node.tagName.toLowerCase() == 'textarea'
	    || node.classList.indexOf('ace_editor') > -1) {
		return;
	}

	switch ( node.nodeType )  
	{
		case 1:  // Element
		case 9:  // Document
		case 11: // Document fragment
			child = node.firstChild;
			while ( child ) 
			{
				next = child.nextSibling;
				walk(child);
				child = next;
			}
			break;

		case 3: // Text node
			handleText(node);
			break;
	}
}

function handleText(textNode) 
{
	var v = textNode.nodeValue;

	v = v.replace(/\bTrump\b/g, "Drumpf");

	textNode.nodeValue = v;
}


})();

manifest.json

{
  "name": "Victor",
  "version": "0.0.1",
  "manifest_version": 2,
  "description": "Injecting stuff",
  "homepage_url": "http://www.houseofagile.com",
  "background": {
    "scripts": [
      "background.js"
    ],
    "persistent": true
  },
  "browser_action": {
    "default_title": "Inject!"
  },
  "permissions": [
    "https://*/*",
    "http://*/*",
    "tabs"
  ]
}