chrome-extension: do not fail if damagedRects property is not available (#11)

* chrome-extension: do not fail if damagedRects property is not available

The ContainerTiming performance entry damagedRects property is a proposal
for the future, and is not available, at least, in the initial patch sent
for review upstream.

Check if the property is available, and if not, do not show the damaged
rects overlay.

* chrome-extension: improve the extension description

Author: jdapena

chrome-extension/package.json

@@ -1,7 +1,7 @@
 {
   "name": "container-timing",
   "version": "0.1.0",
-  "description": "My Chrome Extension",
+  "description": "ContainerTiming Chrome demo extension",
   "private": true,
   "scripts": {
     "watch": "node ./build.mjs --watch",

chrome-extension/src/content.js

@@ -28,7 +28,9 @@ function startPerformanceObserve(elm) {
     console.log("Container timing entries from " + href)
     list.getEntries().forEach((list) => {
       clearRects();
-      showRectsOnScreen(list.damagedRects);
+      if (list?.damagedRects) {
+        showRectsOnScreen(list.damagedRects);
+      }
       showBoundingRect(list.intersectionRect);
       clearRects(true)
     })