Update page action icon based on state

background.js

 			], actions: [ new chrome.declarativeContent.ShowPageAction() ]
 		}]);
 	});
+});
+
+chrome.runtime.onMessage.addListener((request, sender, sendResponse) => {
+	let icons = request.killingStickies ? {
+			"16":	"images/ASK_on_16.png",
+			"32":	"images/ASK_on_32.png",
+			"48":	"images/ASK_on_48.png",
+			"128":	"images/ASK_on_128.png"
+	} : {
+			"16":	"images/ASK_off_16.png",
+			"32":	"images/ASK_off_32.png",
+			"48":	"images/ASK_off_48.png",
+			"128":	"images/ASK_off_128.png"
+	};
+	chrome.pageAction.setIcon({
+		path: icons,
+		tabId: sender.tab.id
+	});
 });

contentScript.js

 	return shouldKillSticky;
 }
 
-function updateIcon(result) {
-	// TODO: code!
+function updateIcon(shouldKillSticky) {
+	chrome.runtime.sendMessage({ "killingStickies" : shouldKillSticky });
 }
 
 /******************/
 	chrome.storage.sync.get([ "matchingPatterns", "exclusionPatterns", "mode" ],
 	(result) => {
 		let shouldKillSticky = checkForShouldKillSticky(result);
-		updateIcon(result);
+		updateIcon(shouldKillSticky);
 		window.onload = () => {
 			if (shouldKillSticky) killSticky();
 		};

manifest.json

 		"default_popup":	"popup.html",
 		"default_title":	"AlwaysKillSticky - Click the icon to control sticky-killing on this site!",
 		"default_icon":		{
-			"16":				"images/ASK16.png",
-			"32":				"images/ASK32.png",
-			"48":				"images/ASK48.png",
-			"128":				"images/ASK128.png"
+			"16":				"images/ASK_on_16.png",
+			"32":				"images/ASK_on_32.png",
+			"48":				"images/ASK_on_48.png",
+			"128":				"images/ASK_on_128.png"
 		}
 	},
 	"icons":			{
-		"16": "images/ASK16.png",
-		"32": "images/ASK32.png",
-		"48": "images/ASK48.png",
-		"128": "images/ASK128.png"
+		"16": "images/ASK_on_16.png",
+		"32": "images/ASK_on_32.png",
+		"48": "images/ASK_on_48.png",
+		"128": "images/ASK_on_128.png"
 	},
 	"options_page":		"options.html"
 }