Fix style sharing bug in TextAutosizer::setMultiplier

TextAutosizing is setting styles on renderers without marking them as
unique which can result in another element sharing the style and getting
an unexpected autosizing value.

I can't find a repro of this, but the code is wrong by inspection. We
really need to make it so that the default version of RenderStyle::clone
makes a unique style and a special one makes a sharable style so these
bugs don't happen.

Review URL: https://codereview.chromium.org/68373005

git-svn-id: svn://svn.chromium.org/blink/trunk@161830 bbb929c8-8fbe-4397-9dbb-9b2b20218538