[Mv2] Add Model.canonicalURLThatCreatedThisTrace
[Mv2] Add Model.canonicalURLThatCreatedThisTrace

This is part of the effort to incrementally land Mv2 code concepts off of the
branch. Might as well start simple.

BUG=catapult:#1885
R=oysteine@chromium.org

Review URL: https://codereview.chromium.org/1582973003

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