Improve RenderBlockFlow::adjustLogicalLineTopAndLogicalHeightIfNeeded

This is just a minor refactoring. First, it makes adjustLogicalLineTopAndLogicalHeightIfNeeded nicer, second
the code will match with WebKit's implementation.

Review URL: https://codereview.chromium.org/38883002

git-svn-id: svn://svn.chromium.org/blink/trunk@160472 bbb929c8-8fbe-4397-9dbb-9b2b20218538