PPAPI: Add missing const to AudioConfig::sample_frame_count().

ppapi/cpp/audio_config.h

Index: ppapi/cpp/audio_config.h
diff --git a/ppapi/cpp/audio_config.h b/ppapi/cpp/audio_config.h
index 3b1cefd27b6c20c7705d5f38ab9828af2629f64c..c3b93c332ec23ae8333c1ef2c4a5eb0764bb063a 100644
--- a/ppapi/cpp/audio_config.h
+++ b/ppapi/cpp/audio_config.h
@@ -120,7 +120,7 @@ class AudioConfig : public Resource {
   /// Getter function for returning the internal sample frame count.
   ///
   /// @return A uint32_t containing the sample frame count.
-  uint32_t sample_frame_count() { return sample_frame_count_; }
+  uint32_t sample_frame_count() const { return sample_frame_count_; }
 
  private:
   PP_AudioSampleRate sample_rate_;