Mark a few more virtual/threaded tests as flaky on win debug

Mark the following layout test as flaky on windows debug builds:
  virtual/threaded/animations/animation-hit-test-transform.html
  virtual/threaded/transitions/blendmode-transitions.html
  virtual/threaded/transitions/cubic-bezier-overflow-svg-length.html
  virtual/threaded/animations/compositor-start-event-timing.html

BUG=248938
TBR=ajuma@chromium.org

Review URL: https://codereview.chromium.org/208393018

git-svn-id: svn://svn.chromium.org/blink/trunk@169869 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix bug in scrollParent

We were early'ing out, but not calling cache->setScrollParent. The result: the
cache thought it was dirty, but it wasn't. This CL fixes that.

BUG=TBD

Review URL: https://codereview.chromium.org/209663007

git-svn-id: svn://svn.chromium.org/blink/trunk@169870 bbb929c8-8fbe-4397-9dbb-9b2b20218538