Teach V8GCController about HTML Imports

In terms of wrapper lifecycle, an imported document is a child of
the master document (or some ancestor imports). This change tells
that to V8GCController so that it can form correct object groups.

R=haraken@chromium.org, adamk@chromium.org
TEST=import-expando-gc.html
BUG=379267

Review URL: https://codereview.chromium.org/309723002

git-svn-id: svn://svn.chromium.org/blink/trunk@175322 bbb929c8-8fbe-4397-9dbb-9b2b20218538