Switch base::Value typemapping to be by value instead of by unique_ptr.

media/base/video_frame_metadata.cc

Index: media/base/video_frame_metadata.cc
diff --git a/media/base/video_frame_metadata.cc b/media/base/video_frame_metadata.cc
index 0ce0b2cdc588c01c20a596f772711c2053a8ed40..58ce920553cfac133db44eced90a3136e4104c0c 100644
--- a/media/base/video_frame_metadata.cc
+++ b/media/base/video_frame_metadata.cc
@@ -151,9 +151,8 @@ bool VideoFrameMetadata::IsTrue(Key key) const {
   return GetBoolean(key, &value) && value;
 }
 
-std::unique_ptr<base::DictionaryValue> VideoFrameMetadata::CopyInternalValues()
-    const {
-  return dictionary_.CreateDeepCopy();
+const base::DictionaryValue& VideoFrameMetadata::GetInternalValues() const {
+  return dictionary_;
 }
 
 void VideoFrameMetadata::MergeInternalValuesFrom(