compositor: Always send an animating tick when a pipeline starts animating (#37507)

Instead of taking into account whether the entire WebView starts
animating, always send an animation tick when a pipeline moves from the
"not animating" to "animating" state. It could be that the WebView was
animating, but not painting if the animation was not producing display
lists. In that case, the required tick would never come, because it is
sent after a repaint.

Testing: Added a new WPT test.
Fixes: #37458.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>

tests/wpt/meta/MANIFEST.json

@@ -454483,6 +454483,10 @@
        "59843ae54b64f6ce4f7e616d4be491c911ea84cf",
        []
       ],
+      "transition-in-iframe-001-iframe.html": [
+       "b7214bd378bd60b2729bc276bd775a389fd4ebcb",
+       []
+      ],
       "two.gif": [
        "01435c80209d533dc2164ac48279574c7ba4615e",
        []
@@ -614633,6 +614637,13 @@
        {}
       ]
      ],
+     "transition-in-iframe-001.html": [
+      "2e5e64de2308aa0c1f176c1d047cd178cac5d3ed",
+      [
+       null,
+       {}
+      ]
+     ],
      "transition-property-001.html": [
       "47a1417070f0c2e7b8171259d9c4f63c44e96bcc",
       [