data-ad-slot attribute is a placeholder for brave ads

As long as the size of a div with that data attribute is set, we will
consider it for ad insertion.

We dont have coverage for tests for this yet but I added this to track
that:
#133

Parent task:
https://app.asana.com/0/41575558488236/76420706546970

Auditors: @bridiver

app/content/webviewPreload.js

@@ -163,7 +163,8 @@ ipc.on(messages.SET_AD_DIV_CANDIDATES, function (e, adDivCandidates, placeholder
   // Common selectors which could be on every page
   var commonSelectors = [
     '[id^="google_ads_iframe_"][id$="__container__"]',
-    '[id^="ad-slot-banner-"]'
+    '[id^="ad-slot-banner-"]',
+    '[data-ad-slot]'
   ]
   commonSelectors.forEach(commonSelector => {
     var nodes = document.querySelectorAll(commonSelector)

js/components/frame.js

@@ -169,11 +169,11 @@ class Frame extends ImmutableComponent {
 
   insertAds (currentLocation) {
     let host = new window.URL(currentLocation).hostname.replace('www.', '')
-    let adDivCandidates = adInfo[host]
-    if (adDivCandidates) {
-      this.webview.send(messages.SET_AD_DIV_CANDIDATES,
-                        adDivCandidates, Config.vault.replacementUrl)
-    }
+    let adDivCandidates = adInfo[host] || []
+    // Call this even when there are no matches because we have some logic
+    // to replace common divs.
+    this.webview.send(messages.SET_AD_DIV_CANDIDATES,
+                      adDivCandidates, Config.vault.replacementUrl)
   }
 
   goBack () {