chore(DASH): Add comments to latest DashParser changes (#8888)

Follow up to #8884 based on comment
#8884 (comment)

Author: tykus160

lib/dash/dash_parser.js

@@ -633,6 +633,8 @@ shaka.dash.DashParser = class {
         }
       }
     }
+    // Clean the array instead of creating a new one. By doing this we ensure
+    // that references to the array does not change in callback functions.
     this.lastCalculatedBaseUris_.splice(0);
     if (!someLocationValid || !this.contentSteeringManager_) {
       const uris = uriObjs.map(TXml.getContents);
@@ -641,13 +643,18 @@ shaka.dash.DashParser = class {
               manifestBaseUris, uris));
     }
 
+    // Here we are creating local variables to avoid direct references to `this`
+    // in a callback function. By doing this we can ensure that garbage
+    // collector can clean up `this` object when it is no longer needed.
     const contentSteeringManager = this.contentSteeringManager_;
     const lastCalculatedBaseUris = this.lastCalculatedBaseUris_;
 
     const getBaseUris = () => {
       if (contentSteeringManager && someLocationValid) {
         return contentSteeringManager.getLocations('BaseURL');
       }
+      // Return the copy, because caller of this function is not an owner
+      // of the array.
       return lastCalculatedBaseUris.slice();
     };
 
@@ -2785,6 +2792,9 @@ shaka.dash.DashParser = class {
       calculatedBaseUris = uriObjs.map(TXml.getContents);
     }
 
+    // Here we are creating local variable to avoid direct references to `this`
+    // in a callback function. By doing this we can ensure that garbage
+    // collector can clean up `this` object when it is no longer needed.
     const contentSteeringManager = this.contentSteeringManager_;
 
     const getFrameUris = () => {