Bug 1317209 - Part 3: Add TElement::get_animation_rule. r=heycam

We use TElement::get_animation_rule to get the animation rules from
Gecko side, which contain the interpolated values of the specific
element and the pseudo type.

components/style/gecko_bindings/bindings.rs

@@ -501,6 +501,11 @@ extern "C" {
     pub fn Gecko_GetServoDeclarationBlock(element: RawGeckoElementBorrowed)
      -> RawServoDeclarationBlockStrongBorrowedOrNull;
 }
+extern "C" {
+    pub fn Gecko_GetAnimationRule(element: RawGeckoElementBorrowed,
+                                  aAtom: *mut nsIAtom)
+     -> RawServoDeclarationBlockStrong;
+}
 extern "C" {
     pub fn Gecko_Atomize(aString: *const ::std::os::raw::c_char, aLength: u32)
      -> *mut nsIAtom;