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>

tests/wpt/mozilla/meta/MANIFEST.json

@@ -12757,15 +12757,6 @@
       {}
      ]
     ],
-    "keyboard-scrolling.html": [
-     "2d9a0c40272d8af49f26de8dc49283df68b2d7b0",
-     [
-      null,
-      {
-       "testdriver": true
-      }
-     ]
-    ],
     "matchMedia.html": [
      "45a7ea268b1ebdba69e947b79d675cc9221428d4",
      [
@@ -13811,6 +13802,24 @@
       {}
      ]
     ],
+    "keyboard-scrolling-iframe.sub.html": [
+     "e002eb6f2a35ed4f73a079acb05b3d99eb04813b",
+     [
+      null,
+      {
+       "testdriver": true
+      }
+     ]
+    ],
+    "keyboard-scrolling.html": [
+     "45916c5d11f351f0ca10cde5e52d5ee60c11bd9d",
+     [
+      null,
+      {
+       "testdriver": true
+      }
+     ]
+    ],
     "keyframe-infinite-percentage.html": [
      "36ba83eeac401653356fa38edf30c94d38fd8542",
      [