Enable promote-img-in-viewport-priority layout test.

This was disabled while the intra-priority changes were landing in chrome, since they modified the output. Disabled in: https://codereview.chromium.org/222553008/

BUG=

Review URL: https://codereview.chromium.org/281363002

git-svn-id: svn://svn.chromium.org/blink/trunk@174265 bbb929c8-8fbe-4397-9dbb-9b2b20218538