Devtools Animations: Display keyframes for CSS Animations in inspector

This change displays CSS animation keyframes alongside the associated
animation player in the Animations sidebar pane. No visible changes are
introduced except with the hidden 'Animation Inspection' experiment enabled.

BUG=419269

Review URL: https://codereview.chromium.org/664993002

git-svn-id: svn://svn.chromium.org/blink/trunk@184328 bbb929c8-8fbe-4397-9dbb-9b2b20218538