DevTools: [Styles] Add "px" suffix to numbers on arrow up/down

This patch adds functionality to proactively add "px" suffix if editing
happens in certain CSS properties.

List of properties where only distance of length values are allowed is
taken from http://www.w3.org/TR/CSS21/propidx.html

BUG=387884

Review URL: https://codereview.chromium.org/343343003

git-svn-id: svn://svn.chromium.org/blink/trunk@177050 bbb929c8-8fbe-4397-9dbb-9b2b20218538