servo: Merge #8564 - Implement Get Element Attribute WebDriver command (from jgraham:webdriver_attr); r=Ms2ger

This intentionally doesn't implement the special handling for boolean
attributes yet, since that requires some kind of exhaustive list of all
such attributes

Source-Repo: https://github.com/servo/servo
Source-Revision: 7f95693288eac546b7b31a51a6fcf80883ae533b

servo/components/msg/webdriver_msg.rs

@@ -15,6 +15,7 @@ pub enum WebDriverScriptCommand {
     FindElementsCSS(String, IpcSender<Result<Vec<String>, ()>>),
     FocusElement(String, IpcSender<Result<(), ()>>),
     GetActiveElement(IpcSender<Option<String>>),
+    GetElementAttribute(String, String, IpcSender<Result<Option<String>, ()>>),
     GetElementTagName(String, IpcSender<Result<String, ()>>),
     GetElementText(String, IpcSender<Result<String, ()>>),
     GetFrameId(WebDriverFrameId, IpcSender<Result<Option<PipelineId>, ()>>),