HTML Imports crashes when inspector is shown.

Inspector expects each resource has originating DocumentLoader object.
But imported document doesn't have any DocumentLoader, which contradict
this assumption. In such case, we could use DocumentLoader of the master document.

BUG=273294
TEST=import-opening-inspector.html
R=dglazkov

Review URL: https://chromiumcodereview.appspot.com/22999014

git-svn-id: svn://svn.chromium.org/blink/trunk@156199 bbb929c8-8fbe-4397-9dbb-9b2b20218538