[BUG] browser.newContext: Unknown permission: clipboard-read for webkit

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**Context:**
- Playwright Version: 1.20.1
- Operating System: Mac
- Node.js version: 14.6
- Browser: webkit

**Code Snippet**
```
const config: PlaywrightTestConfig = {
globalTeardown: require.resolve('../src/utils/teardown'),
workers: configList.workers,
use: {
contextOptions: {
userAgent: configList.userAgent,
permissions: ['clipboard-read', 'clipboard-write']
},
```

```
test('test', async ({ browserName, page }) => {
expect(await page.screenshot({ fullPage: true, timeout: 4000 })).toMatchSnapshot(`${browserName}-EN.png`);
});
```

**Describe the bug**

Test is failing for webkit browser, if config file contains `permissions: ['clipboard-read', 'clipboard-write']` however there is not any clipboard event is being used in this test

error :
```
3) [Desktop Firefox] › search/.test.ts:38:5 › Landing page Test Suite › test

browser.newContext: Unknown permission: clipboard-read
```

Start with the permissions configuration shown in search/.test.ts and reproduce the failure using browser.newContext with WebKit and ['clipboard-read', 'clipboard-write']. Trace how these permissions are validated for WebKit, then add or update coverage so the intended behavior is explicit and the failing configuration no longer produces an unknown-permission error.

Written by the indexing model from the issue text.