fix(ads): Add ResizeObserver to CS ad manager. (#3652)

Closes: #3638

lib/ads/client_side_ad_manager.js

@@ -32,6 +32,9 @@ shaka.ads.ClientSideAdManager = class {
     /** @private {HTMLMediaElement} */
     this.video_ = video;
 
+    /** @private {ResizeObserver} */
+    this.resizeObserver_ = null;
+
     /** @private {number} */
     this.requestAdsStartTime_ = NaN;
 
@@ -180,6 +183,16 @@ shaka.ads.ClientSideAdManager = class {
             this.video_.offsetHeight, viewMode);
       });
 
+      if ('ResizeObserver' in window) {
+        this.resizeObserver_ = new ResizeObserver(() => {
+          const viewMode = document.fullscreenElement ?
+              google.ima.ViewMode.FULLSCREEN : google.ima.ViewMode.NORMAL;
+          this.imaAdsManager_.resize(this.video_.offsetWidth,
+              this.video_.offsetHeight, viewMode);
+        });
+        this.resizeObserver_.observe(this.video_);
+      }
+
       // Single video and overlay ads will start at this time
       // TODO (ismena): Need a better inderstanding of what this does.
       // The docs say it's called to 'start playing the ads,' but I haven't