Add defaulted move constructors for some types that just got copy constructors

They can all benefit from moving, since they contain std::string and
std::vector. We intended to add these in
https://codereview.webrtc.org/1896953004/, but got compiler errors we
couldn't make sense of, so we skipped them. It turns out that what the
compiler was complaining about was that when we said we'd have a
user-defined move constructor, it stopped generating a copy assignment
operator for us. This CL solves the problem by outfitting the types
with defaulted copy and move assignment operators too.

Review URL: https://codereview.webrtc.org/1899173002

Cr-Commit-Position: refs/heads/master@{#12469}

webrtc/modules/audio_coding/codecs/audio_encoder.cc

@@ -16,10 +16,13 @@
 namespace webrtc {
 
 AudioEncoder::EncodedInfo::EncodedInfo() = default;
-
 AudioEncoder::EncodedInfo::EncodedInfo(const EncodedInfo&) = default;
-
+AudioEncoder::EncodedInfo::EncodedInfo(EncodedInfo&&) = default;
 AudioEncoder::EncodedInfo::~EncodedInfo() = default;
+AudioEncoder::EncodedInfo& AudioEncoder::EncodedInfo::operator=(
+    const EncodedInfo&) = default;
+AudioEncoder::EncodedInfo& AudioEncoder::EncodedInfo::operator=(EncodedInfo&&) =
+    default;
 
 int AudioEncoder::RtpTimestampRateHz() const {
   return SampleRateHz();

webrtc/modules/audio_coding/codecs/audio_encoder.h

@@ -46,7 +46,10 @@ class AudioEncoder {
   struct EncodedInfo : public EncodedInfoLeaf {
     EncodedInfo();
     EncodedInfo(const EncodedInfo&);
+    EncodedInfo(EncodedInfo&&);
     ~EncodedInfo();
+    EncodedInfo& operator=(const EncodedInfo&);
+    EncodedInfo& operator=(EncodedInfo&&);
 
     std::vector<EncodedInfoLeaf> redundant;
   };