Improve neutered adsbygoogle scriptlet

Related issue:
- uBlockOrigin/uBlock-issues#786

A case of web page embedding multiple times the
`adsbygoogle.js` script was causing the neutered,
replacement script to create a huge amount of iframes
in the DOM.

The scriptlet has been modified to check if an iframe
tag already exist and skip the creation if so.

Related commits:
- gorhill/uBlock@0934779
- gorhill/uBlock@5fa8739

Co-authored-by: Raymond Hill <rhill@raymondhill.net>

assets/resources/resources.txt

@@ -591,7 +591,7 @@ googlesyndication.com/adsbygoogle.js application/javascript
 	window.adsbygoogle = window.adsbygoogle || {
 		length: 0,
 		loaded: true,
-		push: function Si(a) {
+		push: function Si() {
 			/*
 			client = client || google_ad_client || google_ad_client;
 			slotname = slotname || google_ad_slot;
@@ -600,16 +600,18 @@ googlesyndication.com/adsbygoogle.js application/javascript
 			this.length += 1;
 		}
 	};
-	var phs = document.querySelectorAll('.adsbygoogle');
-	var css = 'height:1px!important;max-height:1px!important;max-width:1px!important;width:1px!important;';
-	for ( var i = 0; i < phs.length; i++ ) {
-		var fr = document.createElement('iframe');
-		fr.id = 'aswift_' + (i+1);
+	const phs = document.querySelectorAll('.adsbygoogle');
+	const css = 'height:1px!important;max-height:1px!important;max-width:1px!important;width:1px!important;';
+	for ( let i = 0; i < phs.length; i++ ) {
+		const id = `aswift_${(i+1)}`;
+		if ( document.querySelector(`iframe#${id}`) !== null ) { continue; }
+		const fr = document.createElement('iframe');
+		fr.id = id;
 		fr.style = css;
-		var cfr = document.createElement('iframe');
-		cfr.id = 'google_ads_frame' + i;
+		const cfr = document.createElement('iframe');
+		cfr.id = `google_ads_frame${i}`;
 		fr.appendChild(cfr);
-		document.body.appendChild(fr);
+		phs[i].appendChild(fr);
 	}
 })();