[HTML Imports] Add a testcase for "element removed flag".

This change adds a test for "element removed flag" concept in the spec:
https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/imports/index.html#dfn-element-removed-flag

Blink already behaves in this way and this test is just for capturing
current behavior.

TEST=import-element-removed-flag.html
R=dglazkov@chromium.org,rafaelw@chromium.org
BUG=302312

Review URL: https://codereview.chromium.org/26230002

git-svn-id: svn://svn.chromium.org/blink/trunk@159038 bbb929c8-8fbe-4397-9dbb-9b2b20218538