Blink CompositorAnimations: Do not crash if no compositor animation player.

blink::Animation can loose compositorPlayer if Animation::m_content detached via
Animation::setEffect(nullptr)

If KeyframeEffect::cancelAnimationOnCompositor wants to clear m_compositorAnimationIds
then it shouldn't crash.

Also, note the existing comment in CompositorAnimations::cancelAnimationOnCompositor (above changed lines)

BUG=571504
R=dstockwell@chromium.org

Review URL: https://codereview.chromium.org/1543143002

Cr-Commit-Position: refs/heads/master@{#367055}

third_party/WebKit/Source/core/animation/CompositorAnimations.cpp

@@ -383,8 +383,8 @@ void CompositorAnimations::cancelAnimationOnCompositor(const Element& element, c
     }
     if (RuntimeEnabledFeatures::compositorAnimationTimelinesEnabled()) {
         WebCompositorAnimationPlayer* compositorPlayer = animation.compositorPlayer();
-        ASSERT(compositorPlayer);
-        compositorPlayer->removeAnimation(id);
+        if (compositorPlayer)
+            compositorPlayer->removeAnimation(id);
     } else {
         toLayoutBoxModelObject(element.layoutObject())->layer()->compositedLayerMapping()->mainGraphicsLayer()->removeAnimation(id);
     }