WebDriver+Browser: Implement GET /session/{id}/screenshot

This doesn't follow the spec to a tee. Our OutOfProcessWebView already
has a bitmap that can be used as the window screenshot. Therefore, we
can bypass the steps that assume we need to access the window's frame
buffer in-flight.

We also don't create an HTMLCanvasElement. We would need a Document in
the WebDriver process to do so. Instead, we can still run the encoding
steps exactly as-is using the screenshot bitmap.

Userland/Services/WebDriver/Session.h

@@ -68,6 +68,7 @@ public:
     ErrorOr<JsonValue, WebDriverError> add_cookie(JsonValue const& payload);
     ErrorOr<JsonValue, WebDriverError> delete_cookie(StringView const& name);
     ErrorOr<JsonValue, WebDriverError> delete_all_cookies();
+    ErrorOr<JsonValue, WebDriverError> take_screenshot();
 
 private:
     void delete_cookies(Optional<StringView> const& name = {});