DevTools: [Styles] do not add px suffix to line-height properties

'line-height' property is perfectly valid without a unit identifier, so
do not force adding "px" suffix if there's none.

BUG=387884

Review URL: https://codereview.chromium.org/361873003

git-svn-id: svn://svn.chromium.org/blink/trunk@177293 bbb929c8-8fbe-4397-9dbb-9b2b20218538