Changing between multicol and regular block shouldn't recreate all renderers.

Make the new (flowthread-based) multicol implementation cope without
having to reattach renderers for all nodes in the subtree.

Up until recently (last year) a reattach was only done for the new
flowthread-based multicol implementation, while it wasn't done for the
old implementation (ColumnInfo), but due to some refactoring of the
code that determines whether or not to reattach on style changes, any
changes to column-width or column-count would result in a reattach,
regardless of which implementation was used.

Now that both implementations can handle it, there's no need to force
a reattach anymore.

Review URL: https://codereview.chromium.org/218663004

git-svn-id: svn://svn.chromium.org/blink/trunk@171192 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix LinkHighlighting crash with squashing + universal overflow scroll.

A combination of --enable-layer-squashing --enable-accelerated-overflow-scroll
--enable-universal-accelerated-overflow-scroll --simulate-touch-screen-with-mouse
can cause a crash in LinkHighlight.cpp if we try to access
RenderLayer::compositedLayerMapping() on a squashed layer (ie, when we should really
be instead checking RenderLayer::groupedMapping()).

BUG=356209

Review URL: https://codereview.chromium.org/210083006

git-svn-id: svn://svn.chromium.org/blink/trunk@171193 bbb929c8-8fbe-4397-9dbb-9b2b20218538