Fix Terminal.processId when setProcessId has not yet been called

Part of #12770

src/vs/workbench/api/node/extHostTerminalService.ts

@@ -40,9 +40,11 @@ export class ExtHostTerminal implements vscode.Terminal {
 		if (this._processId) {
 			return Promise.resolve<number>(this._processId);
 		}
-		setTimeout(() => {
-			return this.processId;
-		}, 200);
+		return new Promise<number>((resolve) => {
+			setTimeout(() => {
+				this.processId.then(resolve);
+			}, 200);
+		});
 	}
 
 	public sendText(text: string, addNewLine: boolean = true): void {