Use EnsureMutableInvalidationSet in RuleFeatureSet::AddInvalidationSet

Rather than calling EnsureInvalidationSet() and performing another
lookup in |map|, just use the |slot| that we already have and call
EnsureMutableInvalidationSet().
Also change to move the invalidation set into |slot| if it is empty.

Change-Id: Ibefdf3ac7ef13fd3b7e05e4086405a0b980603f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2289838
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Auto-Submit: Fredrik Söderquist <fs@opera.com>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786838}

third_party/blink/renderer/core/css/rule_feature_set.cc

@@ -321,10 +321,10 @@ void RuleFeatureSet::AddInvalidationSet(
   scoped_refptr<InvalidationSet>& slot =
       map.insert(key, nullptr).stored_value->value;
   if (!slot) {
-    slot = invalidation_set;
+    slot = std::move(invalidation_set);
   } else {
-    EnsureInvalidationSet(
-        map, key, invalidation_set->GetType(),
+    EnsureMutableInvalidationSet(
+        slot, invalidation_set->GetType(),
         invalidation_set->IsSelfInvalidationSet() ? kSubject : kAncestor)
         .Combine(*invalidation_set);
   }
@@ -338,10 +338,10 @@ void RuleFeatureSet::AddInvalidationSet(
   scoped_refptr<InvalidationSet>& slot =
       map.insert(key, nullptr).stored_value->value;
   if (!slot) {
-    slot = invalidation_set;
+    slot = std::move(invalidation_set);
   } else {
-    EnsureInvalidationSet(
-        map, key, invalidation_set->GetType(),
+    EnsureMutableInvalidationSet(
+        slot, invalidation_set->GetType(),
         invalidation_set->IsSelfInvalidationSet() ? kSubject : kAncestor)
         .Combine(*invalidation_set);
   }