This unit test fails with BGPT force-enabled:
out/Debug/webkit_unit_tests --gtest_filter=AnimationCompositorAnimationsTest.canStartElementOnCompositorEffect

../../third_party/blink/renderer/core/animation/compositor_animations_test.cc:1362: Failure
Expected equality of these values:
  host->GetCompositedAnimationsCountForTesting()
    Which is: 0
  1u
    Which is: 1

To force-enable BGPT we need to both set the BlinkGenPropertyTreesEnabled flag, and set the use_layer_lists setting. This can be done by patching in https://chromium-review.googlesource.com/c/chromium/src/+/1058604 but we'll need to make a version of this test that runs in BGPT mode.