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Add custom built-in element support to <embed>, <object>, and <form>.

Bug: 805281, 806184, 806185, 806187
Change-Id: I920ea11ed0b5884c69d3c1aee04e18f7fa62e9f2
Reviewed-on: https://chromium-review.googlesource.com/895234Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533234}