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 27198ec8ab3b0ccb63e084f42facf9ac03de170b..ae607798dca9390a0c1bbc1dbb9d5a04e8a585c9 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());