Remove ComputedStyle check in CheckCanStartEffectOnCompositor.

Now that all of the unit tests construct real elements with a computed
style we should never ask to start an effect on an element without a
computed style.

Bug: 961686
Change-Id: I6ca2148994ed84cd3d4df9e77aae5576a29154d6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1618064
Commit-Queue: Philip Rogers <pdr@chromium.org>
Auto-Submit: Robert Flack <flackr@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661040}

third_party/blink/renderer/core/animation/compositor_animations.cc

@@ -203,10 +203,7 @@ CompositorAnimations::CheckCanStartEffectOnCompositor(
     // Elements with subtrees containing will-change: contents are not
     // composited for animations as if the contents change the tiles
     // would need to be rerastered anyways.
-    // TODO(crbug.com/961686): Remove Style() check once unit tests create
-    // fully styled Elements.
-    if (layout_object && layout_object->Style() &&
-        layout_object->Style()->SubtreeWillChangeContents()) {
+    if (layout_object && layout_object->Style()->SubtreeWillChangeContents()) {
       reasons |= kTargetHasInvalidCompositingState;
     }
   }