audio: Make AudioBus protected section private.

It has no derived classes, and this default constructor
would be bad to call in any event, since it would leave
several fields uninitialized.

Change-Id: I27a90d641607e34812ce4f642dbce1563c9642ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2561732
Auto-Submit: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Reviewed-by: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831877}

third_party/blink/renderer/platform/audio/audio_bus.h

@@ -165,9 +165,7 @@ class PLATFORM_EXPORT AudioBus : public ThreadSafeRefCounted<AudioBus> {
   static scoped_refptr<AudioBus> GetDataResource(int resource_id,
                                                  float sample_rate);
 
- protected:
-  AudioBus() = default;
-
+ private:
   AudioBus(unsigned number_of_channels, uint32_t length, bool allocate);
 
   void DiscreteSumFrom(const AudioBus&);
@@ -182,7 +180,6 @@ class PLATFORM_EXPORT AudioBus : public ThreadSafeRefCounted<AudioBus> {
   int layout_;
   float sample_rate_;  // 0.0 if unknown or N/A
 
- private:
   DISALLOW_COPY_AND_ASSIGN(AudioBus);
 };