Rename isRoot to isDocumentElement.

Root is a very overloaded term. It's not clear what the
documentElement is a root of and people confuse it
with the RenderView.

Also, delete the unused isRoot method from HTMLImport.h.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/233063007

git-svn-id: svn://svn.chromium.org/blink/trunk@171494 bbb929c8-8fbe-4397-9dbb-9b2b20218538