Convert between {Accumulate, Interpolate}Matrix and the related gecko type.

We convert ComputedOperation::{Accumulate, Interpolate}Matrix into gecko type
not on the main thread, so we cannot use nsCSSValueList_heap (which is not
thread safe so we cannot create it and destroy it on different threads).
Therefore, we use nsCSSValueSharedList to represent the cloned
from_list/to_list. In this patch, we also implement the reversing way,
i.e. Convert eCSSKeyword_{accumulate, interpolate}matrix into
{Accumulate, Interpolate}Matrix.

components/style/gecko/generated/bindings.rs

@@ -1310,6 +1310,10 @@ extern "C" {
     pub fn Gecko_CSSValue_SetPairList(css_value: nsCSSValueBorrowedMut,
                                       len: u32);
 }
+extern "C" {
+    pub fn Gecko_CSSValue_InitSharedList(css_value: nsCSSValueBorrowedMut,
+                                         len: u32);
+}
 extern "C" {
     pub fn Gecko_CSSValue_Drop(css_value: nsCSSValueBorrowedMut);
 }