Bug 1303229: Get the proper viewport size for animations. r=hiro,heycam

While we're here, don't re-recompute the default computed values, just grab them
from the device.

MozReview-Commit-ID: GqqnPLIwN2F
Signed-off-by: Emilio Cobos Álvarez <emilio@crisal.io>

components/style/gecko_bindings/bindings.rs

@@ -1373,8 +1373,7 @@ extern "C" {
                                            style: ServoComputedValuesBorrowed,
                                            parent_style:
                                                ServoComputedValuesBorrowedOrNull,
-                                           pres_context:
-                                               RawGeckoPresContextBorrowed,
+                                           set: RawServoStyleSetBorrowed,
                                            result:
                                                RawGeckoComputedKeyframeValuesListBorrowedMut);
 }