Sample Extension for Tabs Zoom API.

chrome/common/extensions/docs/examples/api/tabs/zoom/background.js

+/**
+ * @fileoverview In this extension, the background page demonstrates how to
+ *               listen for zoom change events.
+*/
+
+function zoomChangeListener(zoomChangeInfo) {
+  var settings_str = "mode:" + zoomChangeInfo.zoomSettings.mode +
+      ", scope:" + zoomChangeInfo.zoomSettings.scope;
+
+  console.log('[ZoomDemoExtension] zoomChangeListener(tab=' +
+              zoomChangeInfo.tabId + ', new=' +
+              zoomChangeInfo.newZoomFactor + ', old=' +
+              zoomChangeInfo.oldZoomFactor + ', ' +
+              settings_str + ')');
+}
+
+chrome.tabs.onZoomChange.addListener(zoomChangeListener);