The default render object layout implementation is dead code

While trying to refactor the render tree layout to be non-recursive I discovered
that the default render object layout implementation is dead code.  First, I put
an assert to find a layouttest that exercised it, but could not find any.  Next,
I made the method pure virtual to see what subclasses used the default impl and
found to my surprise that none do!

BUG=331879

Review URL: https://codereview.chromium.org/132703002

git-svn-id: svn://svn.chromium.org/blink/trunk@164846 bbb929c8-8fbe-4397-9dbb-9b2b20218538