HTML Imports: Force imports always stay standard mode.

Implement https://www.w3.org/Bugs/Public/show_bug.cgi?id=24349

TEST=import-mode-standard.html, import-mode-quirks.html
R=adamk@chromium.org, esprehn@chromium.org
BUG=343705

Review URL: https://codereview.chromium.org/243873002

git-svn-id: svn://svn.chromium.org/blink/trunk@171988 bbb929c8-8fbe-4397-9dbb-9b2b20218538