docs: change examples in docs to use for..of and async/await (#15196)

docs/api/desktop-capturer.md

@@ -12,24 +12,28 @@ title is `Electron`:
 // In the renderer process.
 const { desktopCapturer } = require('electron')
 
-desktopCapturer.getSources({ types: ['window', 'screen'] }, (error, sources) => {
+desktopCapturer.getSources({ types: ['window', 'screen'] }, async (error, sources) => {
   if (error) throw error
-  for (let i = 0; i < sources.length; ++i) {
-    if (sources[i].name === 'Electron') {
-      navigator.mediaDevices.getUserMedia({
-        audio: false,
-        video: {
-          mandatory: {
-            chromeMediaSource: 'desktop',
-            chromeMediaSourceId: sources[i].id,
-            minWidth: 1280,
-            maxWidth: 1280,
-            minHeight: 720,
-            maxHeight: 720
+  for (const source of sources) {
+    if (source.name === 'Electron') {
+      try {
+        const stream = await navigator.mediaDevices.getUserMedia({
+          audio: false,
+          video: {
+            mandatory: {
+              chromeMediaSource: 'desktop',
+              chromeMediaSourceId: source.id,
+              minWidth: 1280,
+              maxWidth: 1280,
+              minHeight: 720,
+              maxHeight: 720
+            }
           }
-        }
-      }).then((stream) => handleStream(stream))
-        .catch((e) => handleError(e))
+        })
+        handleStream(stream)
+      } catch (e) {
+        handleError(e)
+      }
       return
     }
   }