Straighten up HTML Imports load vs. share loader logic.

This is just moving code around in preparation for
making HTMLImportController not being a root HTMLImport.

R=morrita
BUG=

Review URL: https://codereview.chromium.org/303643005

git-svn-id: svn://svn.chromium.org/blink/trunk@175002 bbb929c8-8fbe-4397-9dbb-9b2b20218538