Web Animations API: Add getAnimationPlayers()

Add getAnimationPlayers() to the AnimationTimeline and Element.

Spec:
http://dev.w3.org/fxtf/web-animations/#widl-AnimationTimeline-getAnimationPlayers-sequence-AnimationPlayer
http://dev.w3.org/fxtf/web-animations/#widl-Animatable-getAnimationPlayers-sequence-AnimationPlayer

BUG=391364

Review URL: https://codereview.chromium.org/369223002

git-svn-id: svn://svn.chromium.org/blink/trunk@178326 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Revert of "Blink does not respect input.selectionStart and input.selectionEnd for some cases"
    r177724 https://codereview.chromium.org/258063005
Revert of "Auto-rebaseline for r177724"
    r177733 https://codereview.chromium.org/376183003

Reason for revert:
r177724 caused multiple regressions.

TBR=harpreet.sk@samsung.com
NOTRY=true
BUG=133242,367736,393148,393168

Review URL: https://codereview.chromium.org/404483002

git-svn-id: svn://svn.chromium.org/blink/trunk@178327 bbb929c8-8fbe-4397-9dbb-9b2b20218538