Clear sibling floats while splitting inline flow.

During RenderInline::splitFlow(), floats are cleared on an anonymous containingBlock() for
the inline being split. This is a problem if siblings of the block contain references to
the same floats, since the float removal code in markSiblingsWithFloatsForLayout() will not
later find them.

R=inferno@chromium.org
BUG=245727

Review URL: https://codereview.chromium.org/203483002

git-svn-id: svn://svn.chromium.org/blink/trunk@169658 bbb929c8-8fbe-4397-9dbb-9b2b20218538