text-align-last only affect to the last line when text-align is justify.

CSS3 Text spec about text-align-last has been modified
as the property only can affect to the last line when test-align is justify.

http://www.w3.org/TR/css-text-3/#text-align-last-property

BUG=248839

Review URL: https://codereview.chromium.org/59753002

git-svn-id: svn://svn.chromium.org/blink/trunk@179082 bbb929c8-8fbe-4397-9dbb-9b2b20218538