Work-in-progress: MSEIW (cross-context track mgmt)

Makes sourceBuffer {A/V/T}Track attribute language for MSE-in-Worker
normative (formerly was in a non-normative note).

Updates MSE-in-Worker version of the initialization segment received
algorithm to create cross-context track clones on track creation, and to
conditionally update the cross-context media element's readyState.

BIG TODO: Updates removeSourceBuffer language to remove cross-context
track clones on SourceBuffer removal from its parent media source.

BIG TODO: Updates the "Changes to selected/enabled track state" section
to describe cross-context track clone changes. See also w3c#281.

Also fixes a couple minor typos.