refactor(cdk/testing): fix out of sync file (angular#30021)

Fixes a file that isn't in sync with the internal codebase.

Author: crisbeto

src/cdk/testing/protractor/protractor-element.ts

@@ -207,7 +207,11 @@ export class ProtractorElement 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.');
     }