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This is a reland of fe6e27d6

Old ExtensionApiTest.OpenOrClosedShadowRootInBackgroundPage
navigated to unnecessary empty.html and timeout error occurs
as flaky, but TC already runs the background page successfully,
so this CL rewrites the test case using
ExtensionApiTest::RunExtensionTest simply to fix a flaky issue.

Original change's description:
> Allow access to closed shadow root from Extension
>
> This CL introduces chrome.dom.openOrClosedShadowRoot that is a
> read-only property to allow access to the closed shadow root in
> the Extension's content script.
>
> Bug: 778816
> Change-Id: I933bc5d3996a74f46ee75181c535150901a5a46a
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2319951
> Commit-Queue: Miyoung Shin <myid.shin@igalia.com>
> Reviewed-by: Mike West <mkwst@chromium.org>
> Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
> Reviewed-by: Mason Freed <masonfreed@chromium.org>
> Reviewed-by: Kent Tamura <tkent@chromium.org>
> Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#809277}

TBR=peria@chromium.org,yukishiino@chromium.org,tkent@chromium.org,karandeepb@chromium.org,mkwst@chromium.org,masonfreed@chromium.org

Bug: 778816
Change-Id: I3bb7289cbc0c7c8a1c970000d87458f937acd4b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2428528
Commit-Queue: Miyoung Shin <myid.shin@igalia.com>
Reviewed-by: Miyoung Shin <myid.shin@igalia.com>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812578}