Devtools Animations: UI changes

This change cleans up the UI in the Animations pane. The show subtree
animations checkbox is moved to the status bar. The slider is inlined
correctly. Playback rate buttons are now spaced evenly. A label is
also placed to explain the status bar controls.

BUG=419269

Review URL: https://codereview.chromium.org/733053007

git-svn-id: svn://svn.chromium.org/blink/trunk@185505 bbb929c8-8fbe-4397-9dbb-9b2b20218538