Mark VideoFrame::AsHumanReadableString() const.

Not sure why it was not done previously.

R=sandersd

Change-Id: Ifc72071add5ce1a753d4ffab977826a9ac56d964
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2013910
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734189}

media/base/video_frame.cc

@@ -1109,7 +1109,7 @@ gpu::SyncToken VideoFrame::UpdateReleaseSyncToken(SyncTokenClient* client) {
   return release_sync_token_;
 }
 
-std::string VideoFrame::AsHumanReadableString() {
+std::string VideoFrame::AsHumanReadableString() const {
   if (metadata()->IsTrue(VideoFrameMetadata::END_OF_STREAM))
     return "end of stream";
 

media/base/video_frame.h

@@ -544,7 +544,7 @@ class MEDIA_EXPORT VideoFrame : public base::RefCountedThreadSafe<VideoFrame> {
   gpu::SyncToken UpdateReleaseSyncToken(SyncTokenClient* client);
 
   // Returns a human-readable string describing |*this|.
-  std::string AsHumanReadableString();
+  std::string AsHumanReadableString() const;
 
   // Unique identifier for this video frame; generated at construction time and
   // guaranteed to be unique within a single process.