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Use Shadow DOM v1 APIs where possible for layout tests in //shadow-dom
directory, e.g. Event.path -> Event.deepPath(), createShadowRoot({mode: 'open'})
-> attachShadow({mode: 'open'}).

BUG=592371

Review URL: https://codereview.chromium.org/1777833002

Cr-Commit-Position: refs/heads/master@{#380097}