Auto merge of #11205 - pcwalton:avoid-needless-animation-state-changes, r=jdm

script: Avoid needless `ChangeRunningAnimationsState` messages during typical `requestAnimationFrame()` animations.

This skips useless message traffic when `requestAnimationFrame()` is
called during an animation frame callback. It reduces CPU usage of the
following snippet by 49%:

    <script>
        function foo() {
            requestAnimationFrame(foo);
        }
    </script>
    <button onclick="foo()">Start rAF</button>

Partially addresses #9844.

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