Rename confusing float methods on RenderBlock[Flow]

clearFloats clears the floating object set on RenderBlock and rebuilds it
from intruding floats. newLine performs the CSS version of float clearing.
I'm renaming newLine to clearFloats (which matches clearFloatsIfNeeded)
and the current version of clearFloats to rebuildFloatingObjectSetFromIntrudingFloats.

BUG=302060

Review URL: https://codereview.chromium.org/25346004

git-svn-id: svn://svn.chromium.org/blink/trunk@158598 bbb929c8-8fbe-4397-9dbb-9b2b20218538