Devtools Animations: Display keyframes for Web Anim in Animations pane

CSS animation keyframes are shown in the 'Animations' sidebar pane.
Animations created via the Web Animations API are not currently shown.
This change shows the keyframes for Web Animations using the same styles
as CSS animation keyframes. No visible change unless the 'Animation
Inspection' hidden experiment is enabled.

BUG=419269

Review URL: https://codereview.chromium.org/667053004

git-svn-id: svn://svn.chromium.org/blink/trunk@184415 bbb929c8-8fbe-4397-9dbb-9b2b20218538