Fix performance regression from Blink r154194.

Change in Blink r154194 caused perf regression on tests that use many floating
objects.

This change reduces the overhead of clearFloats(), at the cost of leaving
obsolete container references in the floating object container map. These
references are cleared when the float or the containing RenderBlock is
deleted. In most cases when these references are being removed, they will be
re-added again during layout anyway.

BUG=260286
R=inferno@chromium.org

Review URL: https://chromiumcodereview.appspot.com/19240005

git-svn-id: svn://svn.chromium.org/blink/trunk@154327 bbb929c8-8fbe-4397-9dbb-9b2b20218538