Remove Attr.prefix setter.

Attr.prefix is readonly in DOM4. Use counters indicate setting
Attr.prefix isn't used at all.

BUG=305105

Review URL: https://codereview.chromium.org/137733003

git-svn-id: svn://svn.chromium.org/blink/trunk@165058 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Gardening. Mark animations/compositor-start-event-timing flaky in legacy-animations-engine.

NOTRY=true
TBR=timloh@chromium.org
BUG=334225

Review URL: https://codereview.chromium.org/138083002

git-svn-id: svn://svn.chromium.org/blink/trunk@165059 bbb929c8-8fbe-4397-9dbb-9b2b20218538