After some discussion and investigation, rough approach currently being considered is:

I really don't want to increase dependency on the already-on-the-path-to-deprecation MediaSource objectUrl. Rather, I'll start considering a more explicit MediaSourceHandle object that's transferable, and also usable via HTMLMediaElement's srcObject set/get. This would allow more flexibility to web authors: they can create the Handle in either the worker or main context, use it to attach to the element, and get the actual MediaSource object from the handle in the specific context from which they need access to it. For some simplicity, I'll probably propose the handle can only provide the MediaSource object *once*, so the MediaSource isn't used in more than one context simultaneously.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/606018c1c9f60367cdd58c40de86dc4596c2bb27

commit 606018c1c9f60367cdd58c40de86dc4596c2bb27
Author: Matt Wolenetz <wolenetz@chromium.org>
Date: Fri Jan 18 19:44:48 2019

MSE-in-workers prep: Simplify MediaSource::Seekable()

Reduce potential cross-thread complexity when MediaSource is owned by a
worker context by eliminating cross-object calls from
MediaSource::Seekable() through HTMLMediaElement::buffered() and back to
MediaSource::Buffered().

BUG=878133

Change-Id: I0f89d5ba91ce20fdaea96a505c1ff5943f260f46
Reviewed-on: https://chromium-review.googlesource.com/c/1415438
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624243}
[modify] https://crrev.com/606018c1c9f60367cdd58c40de86dc4596c2bb27/third_party/blink/renderer/modules/mediasource/media_source.cc