Make moveAllChildrenIncludingFloatsTo non-virtual

There's only a single caller of this method (removeChild uses
it when it's possible to merge surrounding anonymous blocks).
Instead of having this method virtual, removeChild's types
are being tightened up, as anonymous blocks are always
RenderBlockFlows.

BUG=302024

Review URL: https://codereview.chromium.org/125583002

git-svn-id: svn://svn.chromium.org/blink/trunk@164610 bbb929c8-8fbe-4397-9dbb-9b2b20218538