Replace string keyframes identifiers with regular identifiers

This change removes all uses of strings as CSS keyframes identifiers
in LayoutTests and DevTools.

Added test to ensure we still support string identifiers for prefixed CSS animations.
LayoutTests/animations/string-keyframes-identifier.html

BUG=423424

Review URL: https://codereview.chromium.org/686693002

git-svn-id: svn://svn.chromium.org/blink/trunk@184615 bbb929c8-8fbe-4397-9dbb-9b2b20218538