Synchronous layout-forcing APIs are not covered by any guideline

[xiaochengh]

(Noticed when reviewing w3ctag/design-reviews#1043)

There exists a class of JS APIs that queries the up-to-date layout result, and if we don't have that, synchronously forces a layout to recompute it. These APIs are known to be easily misused and cause layout thrashing.

Examples include:
Element.offset*, Document.elementFromPoint(), ...

The current guidelines do not provide a guidance for these known troublemakers:

• Be synchronous when appropriate says a API should be synchronously if "the execution time is short and deterministic", but these APIs can incur a long execution time.

• Design asynchronous APIs using Promises says an API might need to be asynchronous if "the user agent may need to do a significant amount of work on another thread, or in another process, before returning the result", but these APIs do a significant amount of work on the same thread.