[Fuchsia] Implement IsPlatformDecoder() in FuchsiaVideoDecoder

Previously FuchsiaVideoDecoder was incorrectly returning false from
IsPlatformDecoder().

Change-Id: Ic845733de35a1e411ae54fe6379085c9581ddb5a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1809981
Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697888}

media/filters/fuchsia/fuchsia_video_decoder.cc

@@ -165,6 +165,7 @@ class FuchsiaVideoDecoder : public VideoDecoder,
 
   // VideoDecoder implementation.
   std::string GetDisplayName() const override;
+  bool IsPlatformDecoder() const override;
   void Initialize(const VideoDecoderConfig& config,
                   bool low_delay,
                   CdmContext* cdm_context,
@@ -303,6 +304,10 @@ std::string FuchsiaVideoDecoder::GetDisplayName() const {
   return "FuchsiaVideoDecoder";
 }
 
+bool FuchsiaVideoDecoder::IsPlatformDecoder() const {
+  return true;
+}
+
 void FuchsiaVideoDecoder::Initialize(const VideoDecoderConfig& config,
                                      bool low_delay,
                                      CdmContext* cdm_context,