Revert 154194 "Manage FloatingObjects using hashmaps"

Bad perf regression.

> Manage FloatingObjects using hashmaps
> 
> Previously, FloatingObject objects are kept in a list referenced
> by a member variable in RenderBlock. There have been many special
> cases in which these objects end up in lists that are not cleared
> properly due to mutations of the render tree, resulting in
> security bugs.
> 
> This patch changes FloatingObject tracking to use 2 global
> hashmaps, one of which replaces the member variable pointer
> in RenderBlock, and the other of which stores container
> references, so that we can easily find all RenderBlocks that
> have a given FloatingObject in their maps.
> 
> R=inferno@chromium.org
> BUG=245727
> 
> Review URL: https://chromiumcodereview.appspot.com/16827004

TBR=kenrb@chromium.org

Review URL: https://codereview.chromium.org/21426002

git-svn-id: svn://svn.chromium.org/blink/trunk@155275 bbb929c8-8fbe-4397-9dbb-9b2b20218538