Make the web-animations engine use the passed in blink::WebFrameTime values.

public/platform/WebFrameTime.h

Index: public/platform/WebFrameTime.h
diff --git a/public/platform/WebFrameTime.h b/public/platform/WebFrameTime.h
index 1050291c23f3db6177e84cc5bb3a7aa9feec7b15..4f4dc76949ba9ef2676341ec544b2c0223e48142 100644
--- a/public/platform/WebFrameTime.h
+++ b/public/platform/WebFrameTime.h
@@ -36,6 +36,15 @@
 namespace blink {
 
 struct WebFrameTime {
+    const static int64_t Undefined = -1;
+
+    WebFrameTime()
+        : lastFrameTime(Undefined)
+        , renderDeadline(Undefined)
+        , displayFrameTime(Undefined)
+        , predictedNextDisplayFrameTime(Undefined)
+    { }
+
     WebFrameTime(double lastFrameTime, double renderDeadline, double displayFrameTime, double predictedNextDisplayFrameTime)
         : lastFrameTime(lastFrameTime)
         , renderDeadline(renderDeadline)
@@ -43,6 +52,15 @@ struct WebFrameTime {
         , predictedNextDisplayFrameTime(predictedNextDisplayFrameTime)
     { }
 
+    bool isNull() const
+    {
+        return (
+            lastFrameTime == Undefined
+            && renderDeadline == Undefined
+            && displayFrameTime == Undefined
+            && predictedNextDisplayFrameTime == Undefined);
+    }
+
     // FIXME(mithro): Upgrade the time in CLOCK_MONOTONIC values to use a
     // TimeTick like class rather than a bare double.