Unship '::-webkit-distributed(...)' functional pseudo element.

We've already shipped '::content'. It's time to unship '::-webkit-distributed'.

The usage of CSSPseudoElementPrefixedDistributed is '~0%'.
http://www.chromestatus.com/metrics/feature/timeline/popularity/99

BUG=268236

Review URL: https://codereview.chromium.org/212123002

git-svn-id: svn://svn.chromium.org/blink/trunk@170094 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Mark gc-dom-tree-lifetime as flaky timeout

TBR=kouhei@chromium.org
BUG=356828

Review URL: https://codereview.chromium.org/212953009

git-svn-id: svn://svn.chromium.org/blink/trunk@170095 bbb929c8-8fbe-4397-9dbb-9b2b20218538