Make FloatingObjects own it's FloatingObject instances

As part of decoupling FloatingObjects from RenderBlock, change
FloatingObjects to properly manage the FloatingObject instances it
contains.

As part of this change, FloatingObject::clone has been renamed to
FloatingObject::unsafeClone to make it obvious that it shouldn't be used
except in specific circumstances and that it is clearly different from
the new FloatingObject::copyToNewContainer method.

Another side effect of this change is that FloatingObject::setRenderer
has been removed now that the RenderBox must be set when the object is
created.

BUG=271774

Review URL: https://chromiumcodereview.appspot.com/23971011

git-svn-id: svn://svn.chromium.org/blink/trunk@157929 bbb929c8-8fbe-4397-9dbb-9b2b20218538