Remove a few passing tests from TestExpectations

virtual/threaded/transitions/position-background-transition.html
w3c/web-platform-tests/shadow-dom/events/retargeting-focus-events/test-002.html
w3c/web-platform-tests/shadow-dom/shadow-trees/lower-boundary-encapsulation/distribution-001.html
w3c/web-platform-tests/shadow-dom/shadow-trees/lower-boundary-encapsulation/distribution-002.html

BUG=330083/328749
TEST=
TBR=zmo@chromium.org

Review URL: https://codereview.chromium.org/199413018

git-svn-id: svn://svn.chromium.org/blink/trunk@169276 bbb929c8-8fbe-4397-9dbb-9b2b20218538