Revert 250185 "Remove --enable-html-imports"

> Remove --enable-html-imports
> 
> The flag is merged into --enable-experimental-web-feaures.
> 
> TEST=none
> BUG=317429
> R=dglazkov@chromium.org,abarth@chromium.org
> 
> Review URL: https://codereview.chromium.org/152043004

TBR=morrita@chromium.org,dglazkov@chromium.org

Review URL: https://codereview.chromium.org/150203006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250237 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250237