Unit tests for DisplayItemList::AsValue.
Unit tests for DisplayItemList::AsValue.

BUG=570424
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1533663002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#365695}

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