HTML Imports: Move HTMLImportLoader ownership to HTMLImportsController

This is a part of refactoring to emphasize HTMLImportLoader
instead of HTMLImport.

By this change, InspectorPage agent is able to traverse loaded
imports wihtout worrying about de-dup.

R=dglazkov@chromium.org,dominicc@chromium.org
TEST=none
BUG=none

Review URL: https://codereview.chromium.org/227973002

git-svn-id: svn://svn.chromium.org/blink/trunk@171088 bbb929c8-8fbe-4397-9dbb-9b2b20218538