Implement new pseudoElement interface on KeyframeEffect

Make target() return the originating element if target_ is a
PseudoElement (BlinkTarget still returns the layout object).

Add pseudoElement() property which returns the target pseudo-selector
if the target is a pseudo-element (null otherwise).
Invalid and unsupported pseudo-elements will still return a selector
but will not animated (i.e. BlinkTarget returns null).

Add pseudoElement option to KeyframeEffect constructor.

Change-Id: Id7d16ea6966c533e56d1170462ff60a79b277fd5
Bug: 981894

web-animations/interfaces/KeyframeEffect/style-change-events.html

@@ -121,6 +121,15 @@
       },
     });
   },
+  pseudoElement: MakeInEffectTest({
+    setup: elem => elem.animate(
+      {opacity: [0.5, 1]},
+      {duration: 100 * MS_PER_SEC, pseudoElement: '::before'}
+    ),
+    test: effect => {
+      effect.pseudoElement = null;
+    },
+  }),
   iterationComposite: UsePropertyTest(effect => {
     // Get iterationComposite
     effect.iterationComposite;