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Update the credential-management WPTs to use JS modules, including for
Mojo JS bindings.

This also refactors the supporting modules to isolate individual
tests from any browser-specific details.

Bug: 1004256
Change-Id: If5428dbd4f371fdc92c62da2a2378e8221c5c658
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2472201Reviewed-by: Michael Moss <mmoss@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#817715}