Simplify text for many conditional cross-context operations (w3c#304)

Following one of the suggested alternatives by @mwatson2 on previous
pull request review [1], this change introduces a "mirror if necessary"
algorithm and uses it to simplfiy multiple places in the spec text where
the attached MediaSource needs to update the state of the attached
HTMLMediaElement. Note, there are several other cross-context pieces of
text left unchanged, since they are specific to either setting up the
conditionally cross-context communication ports, conditionally tearing
them down, or describe how the extended media element buffering or
seekable attributes behave relative to the potentially cross-context and
asynchronous communication of the state used in those extensions.

Note, upcoming PR for privacy and security section addition will also
update (restrict) the flexibility for optimizing cross-context
communication to help mitigate related timing attacks in
implementations.

[1] w3c#282 (review)