Remove default parameter from virtual layoutBlock function

Remove default parameter from virtual layoutBlock function since it is only
called explicitly once on an instance of one particular overload of this method.

BUG=None

Review URL: https://codereview.chromium.org/135043007

git-svn-id: svn://svn.chromium.org/blink/trunk@165254 bbb929c8-8fbe-4397-9dbb-9b2b20218538