Auto merge of #15245 - andreastt:webdriver-caps, r=jdm

Correct capabilities returned by WebDriver server

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These patches aligns Servo’s capabilities with those in the [WebDriver standard](https://w3c.github.io/webdriver/webdriver-spec.html). See each commit’s message for further information.

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- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because _Servo needs more work before it can pass WPT WebDriver tests_

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components/webdriver_server/lib.rs

@@ -303,9 +303,7 @@ impl Handler {
             let mut capabilities = BTreeMap::new();
             capabilities.insert("browserName".to_owned(), "servo".to_json());
             capabilities.insert("browserVersion".to_owned(), "0.0.1".to_json());
-            capabilities.insert("acceptSslCerts".to_owned(), false.to_json());
-            capabilities.insert("takeScreenshot".to_owned(), true.to_json());
-            capabilities.insert("takeElementScreenshot".to_owned(), false.to_json());
+            capabilities.insert("acceptInsecureCerts".to_owned(), false.to_json());
             let rv = Ok(WebDriverResponse::NewSession(
                 NewSessionResponse::new(
                     session.id.to_string(),