Do not recompute client{width,height} the long way in paddingBoxRect

This patch cleans up a case where paddingBoxRect computing
client{width,height} used content{width,height} and then added the padding.
This was silly because content{width,height} is just the
client{width,height} minus the padding.

Review URL: https://codereview.chromium.org/642823003

git-svn-id: svn://svn.chromium.org/blink/trunk@183565 bbb929c8-8fbe-4397-9dbb-9b2b20218538