Tentatively enable HTMLImports flag to ensure its ship-ability.

TEST=none
R=dglazkov,esprehn
BUG=240592

Review URL: https://codereview.chromium.org/245443002

git-svn-id: svn://svn.chromium.org/blink/trunk@172063 bbb929c8-8fbe-4397-9dbb-9b2b20218538