Update selenium-web-driver-element.ts

Author: tatp22

src/cdk/testing/selenium-webdriver/selenium-web-driver-element.ts

@@ -161,7 +161,7 @@ export class SeleniumWebDriverElement implements TestElement {
   async setContenteditableValue(value: string): Promise<void> {
     const contenteditableAttr = await this.getAttribute('contenteditable');
 
-    if (contenteditableAttr !== '' && contenteditableAttr !== 'true') {
+    if (contenteditableAttr !== '' && contenteditableAttr !== 'true' && contenteditableAttr !== 'plaintext-only') {
       throw new Error('setContenteditableValue can only be called on a `contenteditable` element.');
     }