media: Add out-of-line copy ctors for complex classes.

media/base/text_track_config.cc

Index: media/base/text_track_config.cc
diff --git a/media/base/text_track_config.cc b/media/base/text_track_config.cc
index 0d4b11f6ddbd1b7913ce18ac86d49b538605da46..dbd2b2d5c136acde61dc7404dfd7e4964da86ada 100644
--- a/media/base/text_track_config.cc
+++ b/media/base/text_track_config.cc
@@ -20,6 +20,8 @@ TextTrackConfig::TextTrackConfig(TextKind kind,
       id_(id) {
 }
 
+TextTrackConfig::TextTrackConfig(const TextTrackConfig& other) = default;
+
 bool TextTrackConfig::Matches(const TextTrackConfig& config) const {
   return config.kind() == kind_ &&
          config.label() == label_ &&