Prevent quirks mode for srcdoc iframe document

This change locks document mode for srcdoc-made iframe document.
This matches latest HTML standard and Firefox.

R=abarth
TEST~=srcdoc-cannot-be-in-quirks-mode.html
BUG=365005

Review URL: https://codereview.chromium.org/243413005

git-svn-id: svn://svn.chromium.org/blink/trunk@172068 bbb929c8-8fbe-4397-9dbb-9b2b20218538