RenderButton should allow white space children

RenderButton should follow the normal white space creation rules even though it's a
flexible box. This worked before because RenderButton's children is a single anonymous
block instead of it wrapping each child in their own anonymous block.

BUG=260227

Review URL: https://chromiumcodereview.appspot.com/19537011

git-svn-id: svn://svn.chromium.org/blink/trunk@154799 bbb929c8-8fbe-4397-9dbb-9b2b20218538