Revert 166677 "Promote enableHTMLImports flag from test to exper..."

> Promote enableHTMLImports flag from test to experimental.
> 
> This is blink side change to remove --enable-html-imports flag.
> 
> TEST=none
> BUG=317429
> R=dglazkov@chromium.org,esprehn@chromium.org,hayato@chromium.org
> 
> Review URL: https://codereview.chromium.org/157143004

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

Review URL: https://codereview.chromium.org/138163023

git-svn-id: svn://svn.chromium.org/blink/trunk@166877 bbb929c8-8fbe-4397-9dbb-9b2b20218538