Prime PushPullFIFO before starting AudioDestination

Currently AudioDestination starts with an empty PushPullFIFO, so
the first callback from the AudioDeviceThread always ends up with
a bogus "glitch report" on the histogram.

This CL fixes the issue by priming (prefill) the PushPullFIFO to cover
the first callback from the ADT.

Bug: 833766
Change-Id: I17e40df31586c9dd3e3678706d47f08693b96d45
Reviewed-on: https://chromium-review.googlesource.com/1015201Reviewed-by: Raymond Toy <rtoy@chromium.org>
Commit-Queue: Hongchan Choi <hongchan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551504}

third_party/blink/renderer/platform/audio/audio_destination.cc

@@ -82,6 +82,15 @@ AudioDestination::AudioDestination(AudioIOCallback& callback,
   DCHECK(web_audio_device_);
 
   callback_buffer_size_ = web_audio_device_->FramesPerBuffer();
+
+  // Primes the FIFO for the given callback buffer size. This is to prevent
+  // first FIFO pulls from causing "underflow" errors.
+  unsigned priming_render_quanta =
+      ceil(callback_buffer_size_/(float)AudioUtilities::kRenderQuantumFrames);
+  for (unsigned i = 0; i < priming_render_quanta; ++i) {
+    fifo_->Push(render_bus_.get());
+  }
+
   if (!CheckBufferSize()) {
     NOTREACHED();
   }