Add enums to KeyframeEffectOptions.

The KeyframeEffectOptions interface definition used DOMStrings instead
of enums for the "fill" and "direction" members, which differs from the
spec: http://w3c.github.io/web-animations/#the-animationeffecttimingproperties-dictionary
This patch converts them to the specified enums.

BUG=

Review URL: https://codereview.chromium.org/1711543002

Cr-Commit-Position: refs/heads/master@{#376083}

third_party/WebKit/Source/core/animation/KeyframeEffectOptions.idl

@@ -4,18 +4,19 @@
 
 // https://w3c.github.io/web-animations/#the-animationeffecttimingproperties-dictionary
 
+enum FillMode { "none", "forwards", "backwards", "both", "auto" };
+enum PlaybackDirection { "normal", "reverse", "alternate", "alternate-reverse" };
+
 // TODO(dstockwell): Rename this to AnimationEffectTimingProperties.
 dictionary KeyframeEffectOptions {
     double                             delay = 0;
     double                             endDelay = 0;
-    // TODO(dstockwell): The fill type should be FillMode.
-    DOMString                          fill = "auto";
+    FillMode                           fill = "auto";
     double                             iterationStart = 0.0;
     unrestricted double                iterations = 1.0;
     (unrestricted double or DOMString) duration = "auto";
     // TODO(dstockwell): There is no playbackRate member in the spec.
     double                             playbackRate = 1.0;
-    // TODO(dstockwell): The direction type should be PlaybackDirection.
-    DOMString                          direction = "normal";
+    PlaybackDirection                  direction = "normal";
     DOMString                          easing = "linear";
 };