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
Devtools Animations: Styles set by animations shown in computed styles

When an animation is applying styles to an element, the computed styles
currently show the properties as inherited in the Computed styles pane,
making it difficult to find the properties applied by an animation. This
change checks if an animation is present on the selected node and ensures
properties applied by animations do not appear as inherited. The property
also shows which animation applied that style. There are no visible
changes unless the 'Animation Inspection' experiment is enabled.

BUG=413630

Review URL: https://codereview.chromium.org/668963004

git-svn-id: svn://svn.chromium.org/blink/trunk@184617 bbb929c8-8fbe-4397-9dbb-9b2b20218538