Clean up isSpecialElement

Hide isSpecialElement since it is only used in
htmlediting.cpp. And since the two callers pass
a non-null node, turn the null check into an ASSERT.

Review URL: https://codereview.chromium.org/694893003

git-svn-id: svn://svn.chromium.org/blink/trunk@184850 bbb929c8-8fbe-4397-9dbb-9b2b20218538