DevTools: [Styles] Do not add 'px' suffix to line-height property value

This patch adds exception for "line-height" property so that it is not
treated as a length property

BUG=392182
R=apavlov, vsevik

Review URL: https://codereview.chromium.org/375273003

git-svn-id: svn://svn.chromium.org/blink/trunk@177748 bbb929c8-8fbe-4397-9dbb-9b2b20218538