HTMLImport: Add test case simultating polymer workaround

This patch adds a layout test that confirms unintentionally warning if
imported document adds style to the main document by appendChild
after load.
This simulates polymer's workaround for style deprecation:
(Copy <custom-style> styles to main document: https://git.io/vxP3p)

Bug: 523952
Change-Id: I2eb696b26af06000bc07af6ea9eeb87a67179368
Reviewed-on: https://chromium-review.googlesource.com/991655
Reviewed-by: Takayoshi Kochi <kochi@chromium.org>
Commit-Queue: Yoichi Osato <yoichio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547641}
3 files changed