InspectorDOMAgent: Add isChild() check before a cast

This checks HTMLImport::isChild() before cast it through
toHTMLImportChild()

BUG=361017
TEST=none
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/228763005

git-svn-id: svn://svn.chromium.org/blink/trunk@171157 bbb929c8-8fbe-4397-9dbb-9b2b20218538