Add to-string function for H264 profile level id

webrtc/common_video/h264/profile_level_id.h

Index: webrtc/common_video/h264/profile_level_id.h
diff --git a/webrtc/common_video/h264/profile_level_id.h b/webrtc/common_video/h264/profile_level_id.h
index 84f0005785bb04c2394d07d37a3f4b09bb45a81d..44292631cb51212c55d86d598d738d9c276c9c8e 100644
--- a/webrtc/common_video/h264/profile_level_id.h
+++ b/webrtc/common_video/h264/profile_level_id.h
@@ -11,6 +11,8 @@
 #ifndef WEBRTC_COMMON_VIDEO_H264_PROFILE_LEVEL_ID_H_
 #define WEBRTC_COMMON_VIDEO_H264_PROFILE_LEVEL_ID_H_
 
+#include <string>
+
 #include "webrtc/base/optional.h"
 
 namespace webrtc {
@@ -47,6 +49,8 @@ enum Level {
 };
 
 struct ProfileLevelId {
+  ProfileLevelId(Profile profile, Level level)
+      : profile(profile), level(level) {}
   Profile profile;
   Level level;
 };
@@ -56,6 +60,11 @@ struct ProfileLevelId {
 // profile level id.
 rtc::Optional<ProfileLevelId> ParseProfileLevelId(const char* str);
 
+// Returns canonical string representation as three hex bytes of the profile
+// level id, or returns nothing for invalid profile level ids.
+rtc::Optional<std::string> ProfileLevelIdToString(
+    const ProfileLevelId& profile_level_id);
+
 }  // namespace H264
 }  // namespace webrtc