Minor refactoring to avoid redundant calls to RenderObject::style()

We can avoid redundant calls to style() in the api computePreferredLogicalWidths()
by introducing a local variable.

Review URL: https://codereview.chromium.org/238973008

git-svn-id: svn://svn.chromium.org/blink/trunk@171833 bbb929c8-8fbe-4397-9dbb-9b2b20218538