Marked 2 HTML Imports related test as flaky.

TBR=dominicc
BUG=267302
BUG=267296

Review URL: https://codereview.chromium.org/21711002

git-svn-id: svn://svn.chromium.org/blink/trunk@155377 bbb929c8-8fbe-4397-9dbb-9b2b20218538