Bug 1488122 - Factor out type definition for base values hashmap; r=hiro

Differential Revision: https://phabricator.services.mozilla.com/D13935

--HG--
extra : moz-landing-system : lando

dom/animation/KeyframeEffect.cpp

@@ -374,8 +374,7 @@ void KeyframeEffect::EnsureBaseStyles(
     return;
   }
 
-  Maybe<nsRefPtrHashtable<nsUint32HashKey, RawServoAnimationValue>>
-      previousBaseStyles;
+  Maybe<BaseValuesHashmap> previousBaseStyles;
   if (aBaseStylesChanged != nullptr) {
     previousBaseStyles.emplace(std::move(mBaseValues));
   }

dom/animation/KeyframeEffect.h

@@ -402,7 +402,9 @@ class KeyframeEffect : public AnimationEffect {
   // The non-animated values for properties in this effect that contain at
   // least one animation value that is composited with the underlying value
   // (i.e. it uses the additive or accumulate composite mode).
-  nsRefPtrHashtable<nsUint32HashKey, RawServoAnimationValue> mBaseValues;
+  using BaseValuesHashmap =
+      nsRefPtrHashtable<nsUint32HashKey, RawServoAnimationValue>;
+  BaseValuesHashmap mBaseValues;
 
   // True if this effect is in the EffectSet for its target element. This is
   // used as an optimization to avoid unnecessary hashmap lookups on the