Remove false assertion from WebSubstringUtil::attributedSubstringInRange

rootEditableElementOrDocumentElement can return null.

BUG=401007

Review URL: https://codereview.chromium.org/443323002

git-svn-id: svn://svn.chromium.org/blink/trunk@179680 bbb929c8-8fbe-4397-9dbb-9b2b20218538