Added layout test for getComputedStyle with distribution.

BUG=337618
TEST=fast/dom/shadow/getComputedStyle-with-distribution.html

Review URL: https://codereview.chromium.org/240293007

git-svn-id: svn://svn.chromium.org/blink/trunk@175873 bbb929c8-8fbe-4397-9dbb-9b2b20218538