Web Animations: Update Player model to match our interpretation of the spec

This patch updates our model of Player to match more closely the spec's
behaviour. Note that the implementation here is based not directly off
the spec, but off etherpad.mozilla.org/Z0EHDNFfIJ. We believe this is
a clearer interpretation of the spec, and setStartTime behaves nicer
in some cases.

Note that although setPlaybackRate is implemented here, it will not
immediately work once exposed in the bindings as timeToNextEffect is
not currently set up to handle this.

NOTRY=true
BUG=334926

Review URL: https://codereview.chromium.org/148223016

git-svn-id: svn://svn.chromium.org/blink/trunk@166078 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Auto-rebaseline for r166061

http://src.chromium.org/viewvc/blink?view=revision&revision=166061

BUG=335472
TBR=tkent@chromium.org

Review URL: https://codereview.chromium.org/148383015

git-svn-id: svn://svn.chromium.org/blink/trunk@166079 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Auto-rebaseline for r166068

http://src.chromium.org/viewvc/blink?view=revision&revision=166068

BUG=302005
TBR=skobes@chromium.org

Review URL: https://codereview.chromium.org/149093010

git-svn-id: svn://svn.chromium.org/blink/trunk@166080 bbb929c8-8fbe-4397-9dbb-9b2b20218538