Efficient UnmodifiableMultimap forEach

forEach can pass calls directly to the delegate protected
collection because it is a read-only operation.

Fixes #3911
Fixes #3912

RELNOTES=n/a

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=314952594

guava/src/com/google/common/collect/Multimaps.java

@@ -49,6 +49,7 @@
 import java.util.Set;
 import java.util.SortedSet;
 import java.util.Spliterator;
+import java.util.function.BiConsumer;
 import java.util.function.Consumer;
 import java.util.stream.Collector;
 import java.util.stream.Stream;
@@ -694,6 +695,11 @@ public Collection<Entry<K, V>> entries() {
       return result;
     }
 
+    @Override
+    public void forEach(BiConsumer<? super K, ? super V> consumer) {
+      delegate.forEach(checkNotNull(consumer));
+    }
+
     @Override
     public Collection<V> get(K key) {
       return unmodifiableValueCollection(delegate.get(key));