Less dynamic memory allocs on RT thread in MediaStreamWebAudioSource

Reduce the frequency of dynamic memory allocation in
blink::MediaStreamWebAudioSource::ProvideInput()
which runs on a real-time audio thread.

With the fix it's one-off allocation, rather than on each call.

Bug: 1137778
Change-Id: Ibd33bc3fc8715c1cd9f49033476b97138fbfa027
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2471397
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Auto-Submit: Olga Sharonova <olka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818466}

third_party/blink/renderer/platform/mediastream/media_stream_web_audio_source.cc

@@ -54,11 +54,13 @@ void MediaStreamWebAudioSource::ProvideInput(AudioBus* bus,
 
   // Wrap the AudioBus channel data using WebVector.
   uint32_t n = bus->NumberOfChannels();
-  WebVector<float*> web_audio_data(n);
+  if (web_audio_data_.size() != n)
+    web_audio_data_ = WebVector<float*>(static_cast<size_t>(n));
+
   for (uint32_t i = 0; i < n; ++i)
-    web_audio_data[i] = bus->Channel(i)->MutableData();
+    web_audio_data_[i] = bus->Channel(i)->MutableData();
 
-  web_audio_source_provider_->ProvideInput(web_audio_data, frames_to_process);
+  web_audio_source_provider_->ProvideInput(web_audio_data_, frames_to_process);
 }
 
 }  // namespace blink

third_party/blink/renderer/platform/mediastream/media_stream_web_audio_source.h

@@ -54,6 +54,7 @@ class PLATFORM_EXPORT MediaStreamWebAudioSource : public AudioSourceProvider {
   void ProvideInput(AudioBus*, uint32_t frames_to_process) override;
 
   std::unique_ptr<WebAudioSourceProvider> web_audio_source_provider_;
+  WebVector<float*> web_audio_data_;
 
   DISALLOW_COPY_AND_ASSIGN(MediaStreamWebAudioSource);
 };