Revert 172400 "HTML Imports: Teach preloader about HTML Imports."

LayoutTests/http/tests/htmlimports/redirect.html is so falakey:
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&tests=http%2Ftests%2Fhtmlimports%2Fredirect.html

> HTML Imports: Teach preloader about HTML Imports.
> 
> This change tells HTMLPreloadScanner about <link rel=import> so that
> it can preload the resource for the import.
> 
> TEST=import-preload.html
> R=esprehn@chromium.org,dglazkov@chromium.org
> BUG=323096
> 
> Review URL: https://codereview.chromium.org/243793006

TBR=morrita@chromium.org

Review URL: https://codereview.chromium.org/250503002

git-svn-id: svn://svn.chromium.org/blink/trunk@172461 bbb929c8-8fbe-4397-9dbb-9b2b20218538