[web-animations-1] getKeyframes() processing steps need updating after null composite

[graouts]

In KeyframeEffectReadOnly::getKeyframes(), we now say that composite should always be set with a default of null in the non-normative section:

composite
The keyframe-specific composite operation used to combine the values specified in this keyframe with the underlying value.

This member will be null if the composite operation specified on the keyframe effect is being used.

However, in the (normative) processing steps below, we say:

2. Set offset, computedOffset, easing members of output keyframe to the respective values keyframe offset, computed keyframe offset, and keyframe-specific timing function of keyframe.

3. If keyframe has a keyframe-specific composite operation, set composite to that value.

I believe we should get rid of 3. and update 2. to read:

2. Set offset, computedOffset, easing, composite members of output keyframe to the respective values keyframe offset, computed keyframe offset, keyframe-specific timing function and keyframe-specific composite operation of keyframe.

[birtles]

Yeah, this is one of the (many) awkwardnesses of having a separate model and API. The intention was that "null" is just an API concept corresponding to the absence of a keyframe-specific composite operation. However, we already talk about null offset values so we should just talk about null composite operations too.

[birtles]

2625ca2 fixes the obvious grammatical error in that previous commit too.