Update CSS animations in a SequentialTask.

We create the SequentialTask only if:

* We have no old computed values and we have animation name style in the new
  computed values.
* Any animation properties is changed.
* display property is changed from 'none' and we have animation name style.
* display property is changed to 'none'.

In a subsequent patch we skip the SequentialTask if we have no running
animations and the display propery is changed to 'none'.

components/style/gecko_bindings/bindings.rs

@@ -541,6 +541,14 @@ extern "C" {
                                            RawGeckoStyleAnimationListBorrowed)
      -> bool;
 }
+extern "C" {
+    pub fn Gecko_UpdateAnimations(aElement: RawGeckoElementBorrowed,
+                                  aPseudoTagOrNull: *mut nsIAtom,
+                                  aComputedValues:
+                                      ServoComputedValuesBorrowed,
+                                  aParentComputedValues:
+                                      ServoComputedValuesBorrowedOrNull);
+}
 extern "C" {
     pub fn Gecko_Atomize(aString: *const ::std::os::raw::c_char, aLength: u32)
      -> *mut nsIAtom;