Remove virtualized isRoot/isChild from HTMLImport.

We can just test root-ness by the absence of parent.

No behavior changes, just plumbotron.

R=morrita
BUG=

Review URL: https://codereview.chromium.org/302433016

git-svn-id: svn://svn.chromium.org/blink/trunk@175223 bbb929c8-8fbe-4397-9dbb-9b2b20218538