Remove DC component from BeepingSource

The current generator is maintaining a value of 0 when silent and
varying between 0 and 128 when beeping. As it generates unsigned 8 bit
audio, this means it's stuck at the lowest value when silent, rather
than at the mid-point. This has confused a test.

Bug: 892534
Change-Id: I4bf5d676ef58e0443716213a5650baacf3f11b8c
Reviewed-on: https://chromium-review.googlesource.com/c/1430074Reviewed-by: Max Morin <maxmorin@chromium.org>
Commit-Queue: Oskar Sundbom <ossu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626103}

media/audio/simple_sources.cc

@@ -263,7 +263,7 @@ int BeepingSource::OnMoreData(base::TimeDelta /* delay */,
   // Accumulate the time from the last beep.
   interval_from_last_beep_ += base::TimeTicks::Now() - last_callback_time_;
 
-  memset(buffer_.get(), 0, buffer_size_);
+  memset(buffer_.get(), 128, buffer_size_);
   bool should_beep = false;
   BeepContext* beep_context = GetBeepContext();
   if (beep_context->automatic_beep()) {
@@ -291,7 +291,7 @@ int BeepingSource::OnMoreData(base::TimeDelta /* delay */,
     int position = 0;
     while (position + high_bytes <= buffer_size_) {
       // Write high values first.
-      memset(buffer_.get() + position, 128, high_bytes);
+      memset(buffer_.get() + position, 255, high_bytes);
       // Then leave low values in the buffer with |high_bytes|.
       position += high_bytes * 2;
     }