Auto merge of #23581 - georgeroman:implement_get_page_source_wd_command, r=jdm

Implement GetPageSource WebDriver command

<!-- Please describe your changes on the following line: -->

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

<!-- Either: -->
- [X] There are tests for these changes

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/23581)
<!-- Reviewable:end -->

components/script/script_thread.rs

@@ -1902,6 +1902,9 @@ impl ScriptThread {
             WebDriverScriptCommand::GetActiveElement(reply) => {
                 webdriver_handlers::handle_get_active_element(&*documents, pipeline_id, reply)
             },
+            WebDriverScriptCommand::GetPageSource(reply) => {
+                webdriver_handlers::handle_get_page_source(&*documents, pipeline_id, reply)
+            },
             WebDriverScriptCommand::GetCookies(reply) => {
                 webdriver_handlers::handle_get_cookies(&*documents, pipeline_id, reply)
             },