Fix debug crash when compositing keyword animations together

This change makes CSSValueInterpolationType composite by replacement
instead of crashing in debug.

Review URL: https://codereview.chromium.org/1401853002

Cr-Commit-Position: refs/heads/master@{#353940}

third_party/WebKit/LayoutTests/animations/keyword-composition-crash.html

+<script src="../resources/testharness.js"></script>
+<script src="../resources/testharnessreport.js"></script>
+<div id="target"></div>
+<script>
+test(() => {
+  target.animate({position: 'absolute'}, {fill: 'forwards'});
+  target.animate({position: 'static', composite: 'add'}, {fill: 'forwards'});
+  getComputedStyle(target).position;
+}, "Don't crash when compositing keyword animations.");
+</script>

third_party/WebKit/Source/core/animation/CSSValueInterpolationType.h

@@ -29,6 +29,11 @@ public:
         return nullptr;
     }
 
+    void composite(UnderlyingValue& underlyingValue, double underlyingFraction, const InterpolationValue& value) const final
+    {
+        underlyingValue.set(&value);
+    }
+
     void apply(const InterpolableValue&, const NonInterpolableValue*, StyleResolverState&) const final;
 };