fix(youtube-player): startSeconds not applied when using placeholder

Fixes that the `startSeconds` input wasn't doing anything if there's a placeholder. This used to work, but seems to have broken during the transition to using the placeholder.

Fixes angular#29874.

src/dev-app/youtube-player/youtube-player-demo.html

@@ -13,9 +13,11 @@ <h2>Basic Example</h2>
     <div class="demo-video-selection">
       <mat-checkbox [(ngModel)]="disableCookies">Disable cookies</mat-checkbox>
       <mat-checkbox [(ngModel)]="disablePlaceholder">Disable placeholder</mat-checkbox>
+      <mat-checkbox [(ngModel)]="startAt30s">Start at 30s</mat-checkbox>
     </div>
     <youtube-player [videoId]="selectedVideoId"
                     [playerVars]="playerVars"
+                    [startSeconds]="startAt30s ? 30 : 0"
                     [width]="videoWidth"
                     [height]="videoHeight"
                     [disableCookies]="disableCookies"

src/dev-app/youtube-player/youtube-player-demo.ts

@@ -94,6 +94,7 @@ export class YouTubePlayerDemo implements AfterViewInit, OnDestroy {
   videoHeight: number | undefined;
   disableCookies = false;
   disablePlaceholder = false;
+  startAt30s = false;
   placeholderQuality: PlaceholderImageQuality;
 
   constructor() {

src/youtube-player/youtube-player.ts

@@ -597,6 +597,12 @@ export class YouTubePlayer implements AfterViewInit, OnChanges, OnDestroy {
         const state = player.getPlayerState();
         if (state === PlayerState.UNSTARTED || state === PlayerState.CUED || state == null) {
           this._cuePlayer();
+        } else if (playVideo && this.startSeconds && this.startSeconds > 0) {
+          // We have to use `seekTo` when `startSeconds` are specified to simulate it playing from
+          // a specific time. The "proper" way to do it would be to either go through `cueVideoById`
+          // or `playerVars.start`, but at the time of writing both end up resetting the video
+          // to the state as if the user hasn't interacted with it.
+          player.seekTo(this.startSeconds, true);
         }
 
         this._changeDetectorRef.markForCheck();