script: Chain up keyboard scrolling to parent <iframe>s (#39469)

When an `<iframe>` cannot scroll because the size of the frame is
greater than or
equal to the size of page contents, chain up the keyboard scroll
operation to the parent frame.

Testing: A new Servo-only WPT tests is added, though needs to be
manually
run with `--product servodriver`.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Delan Azabani <dazabani@igalia.com>

components/constellation/tracing.rs

@@ -182,6 +182,7 @@ mod from_script {
                 Self::ReportMemory(..) => target!("ReportMemory"),
                 Self::WebDriverInputComplete(..) => target!("WebDriverInputComplete"),
                 Self::FinishJavaScriptEvaluation(..) => target!("FinishJavaScriptEvaluation"),
+                Self::ForwardKeyboardScroll(..) => target!("ForwardKeyboardScroll"),
             }
         }
     }