Reland of Add optional visualization file writers to VideoProcessor tests. (patchset #1 id:1 of https://codereview.webrtc.org/2708103002/ )

Reason for revert:
Necessary calls were "protected" by RTC_DCHECKs, that were optimized away in some release builds.
Replacing the RTC_DCHECKs with EXPECTs.

Original issue's description:
> Revert of Add optional visualization file writers to VideoProcessor tests. (patchset #4 id:220001 of https://codereview.webrtc.org/2700493006/ )
>
> Reason for revert:
> Breaks downstream project.
>
> Original issue's description:
> > Add optional visualization file writers to VideoProcessor tests.
> >
> > The purpose of this visualization CL is to add the ability to record
> > video at the source, after encode, and after decode, in the VideoProcessor
> > tests. These output files can then be replayed and used as a subjective
> > complement to the objective metric plots given by the existing Python
> > plotting script.
> >
> > BUG=webrtc:6634
> >
> > Review-Url: https://codereview.webrtc.org/2700493006
> > Cr-Commit-Position: refs/heads/master@{#16738}
> > Committed: https://chromium.googlesource.com/external/webrtc/+/872104ac41d7764f8676c9ea55555210bea4605c
>
> TBR=asapersson@webrtc.org,sprang@webrtc.org,kjellander@webrtc.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=webrtc:6634
>
> Review-Url: https://codereview.webrtc.org/2708103002
> Cr-Commit-Position: refs/heads/master@{#16745}
> Committed: https://chromium.googlesource.com/external/webrtc/+/2a8135a1741761bd6de52163c0dc35f6eff7c8eb

TBR=asapersson@webrtc.org,sprang@webrtc.org,kjellander@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
BUG=webrtc:6634

Review-Url: https://codereview.webrtc.org/2706123003
Cr-Commit-Position: refs/heads/master@{#16769}
diff --git a/webrtc/test/testsupport/frame_reader.h b/webrtc/test/testsupport/frame_reader.h
index 13800cd..94dd78b 100644
--- a/webrtc/test/testsupport/frame_reader.h
+++ b/webrtc/test/testsupport/frame_reader.h
@@ -46,14 +46,14 @@
   virtual int NumberOfFrames() = 0;
 };
 
-class FrameReaderImpl : public FrameReader {
+class YuvFrameReaderImpl : public FrameReader {
  public:
   // Creates a file handler. The input file is assumed to exist and be readable.
   // Parameters:
   //   input_filename          The file to read from.
   //   width, height           Size of each frame to read.
-  FrameReaderImpl(std::string input_filename, int width, int height);
-  ~FrameReaderImpl() override;
+  YuvFrameReaderImpl(std::string input_filename, int width, int height);
+  ~YuvFrameReaderImpl() override;
   bool Init() override;
   rtc::scoped_refptr<I420Buffer> ReadFrame() override;
   void Close() override;
@@ -61,10 +61,10 @@
   int NumberOfFrames() override;
 
  private:
-  std::string input_filename_;
+  const std::string input_filename_;
   size_t frame_length_in_bytes_;
-  int width_;
-  int height_;
+  const int width_;
+  const int height_;
   int number_of_frames_;
   FILE* input_file_;
 };