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Introduces a public IsolatedConnection helper as a replacement for the
EDK PeerConnection type. A follow-up C will convert existing uses.

Mojo multiprocess tests are converted to use the new type here,
removing the last remnants of EDK dependency from the test multiprocess
test support code.

This also deletes some dead test code and opportunistically converts a
few related uses of the internal EDK ScopedInternalPlatformHandle type
to the newer public PlatformHandle type.

Bug: 844763
Change-Id: Icad97f51d320381483e474c16077ef7844cc2b9d
Reviewed-on: https://chromium-review.googlesource.com/1099269
Commit-Queue: Ken Rockot <rockot@chromium.org>
Reviewed-by: Jay Civelli <jcivelli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567323}