Make http/tests/media/audio-seekable-contains-zero-without-ranges.html work with multibuffers.

media/blink/multibuffer_data_source.cc

Index: media/blink/multibuffer_data_source.cc
diff --git a/media/blink/multibuffer_data_source.cc b/media/blink/multibuffer_data_source.cc
index d86637d474349ac2da9195c431d14b6e81b35f63..2045629d420faf4c443f7d0a74b3bc135665638d 100644
--- a/media/blink/multibuffer_data_source.cc
+++ b/media/blink/multibuffer_data_source.cc
@@ -398,6 +398,16 @@ void MultibufferDataSource::ReadTask() {
     bytes_read =
         static_cast<int>(std::min<int64_t>(available, read_op_->size()));
     bytes_read = reader_->TryRead(read_op_->data(), bytes_read);
+
+    if (bytes_read == 0 && total_bytes_ == kPositionNotSpecified) {
+      // We've reached the end of the file and we didn't know the total size
+      // before. Update the total size so Read()s past the end of the file will
+      // fail like they would if we had known the file size at the beginning.
+      total_bytes_ = reader_->Tell();
+      if (total_bytes_ != kPositionNotSpecified)
+        host_->SetTotalBytes(total_bytes_);
+    }
+
     ReadOperation::Run(std::move(read_op_), bytes_read);
   } else {
     reader_->Wait(1, base::Bind(&MultibufferDataSource::ReadTask,