Currently AnimationEffectTiming::setEasing passes document=nullptr to TimingInput::SetTimingFunction. This has the effect that AnimationInputHelpers::ParseTimingFunction is called with document=nullptr, which means that the timing function string will always be parsed in an insecure context (https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/animation/AnimationInputHelpers.cpp?l=249&rcl=5c21746912005ce434562783d123591dc4719d98).

We should decide what to do here; should we continue to pass nullptr and accept the insecure context, or get the spec to specify what execution context should be used for timing function parsing in this instance (I don't *think* it currently specifies).