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Adds a readonly, boolean-valued, static attribute named
`canConstructInDedicatedWorker` to the MediaSource interface.
Currently, this attribute is only visible to web apps if the
RunTimeEnabledFeature "MediaSourceInWorkers" is enabled. When visible,
this attribute always returns true.

The primary goal of having this attribute is to enable web app's main
thread to proactively determine whether or not MSE is supported from a
dedicated worker context *before* deciding whether or not to create or
try using MSE from such a context.

As an initial example of this use case, the existing MSE-in-Workers
web_tests are updated to use this new attribute's existence and value
to fail-fast rather than potentially flakily fail (e.g. previously, the
...worker-terminate test might flakily pass/fail some of its test cases
on implementations lacking MSE-in-Workers support if the test completed
before handling receipt of error message from worker.)

A further test is added to ensure that, if the attribute is missing or
exists but is not `true`, a dedicated worker does not have ability to
construct a MediaSource instance. See also
https://github.com/w3c/media-source/issues/175 for further discussion
which led to this new attribute.

BUG=878133

Change-Id: I697ca6adc5b5dc65d5c5084ff67a541430a9237b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2487834
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Reviewed-by: Will Cassella <cassew@google.com>
Cr-Commit-Position: refs/heads/master@{#819564}