Fix progress reporting for multibuffers to report progress for every byte we receive.

media/blink/multibuffer_unittest.cc

Index: media/blink/multibuffer_unittest.cc
diff --git a/media/blink/multibuffer_unittest.cc b/media/blink/multibuffer_unittest.cc
index ba1fde5b0542ef5cc889571f6eea318a677a5f4a..c9aa7e055e9a14c793b0ef7ec85598470374ff08 100644
--- a/media/blink/multibuffer_unittest.cc
+++ b/media/blink/multibuffer_unittest.cc
@@ -65,6 +65,7 @@ class FakeMultiBufferDataProvider : public MultiBuffer::DataProvider {
   MultiBufferBlockId Tell() const override { return pos_; }
 
   bool Available() const override { return !fifo_.empty(); }
+  int64_t AvailableBytes() const override { return 0; }
 
   scoped_refptr<DataBuffer> Read() override {
     DCHECK(Available());