Fix multibuffer crashing bug

media/blink/multibuffer_reader.h

Index: media/blink/multibuffer_reader.h
diff --git a/media/blink/multibuffer_reader.h b/media/blink/multibuffer_reader.h
index 403c03978ba69e14a8b33316da20479108831c3d..19e90617f76ae4f07101927bef962d6bbdbc09ef 100644
--- a/media/blink/multibuffer_reader.h
+++ b/media/blink/multibuffer_reader.h
@@ -101,6 +101,9 @@ class MEDIA_BLINK_EXPORT MultiBufferReader
     return block(byte_pos + (1LL << multibuffer_->block_size_shift()) - 1);
   }
 
+  // Unpin previous range, then pin the new range.
+  void PinRange(MultiBuffer::BlockId begin, MultiBuffer::BlockId end);
+
   // Check if wait operation can complete now.
   void CheckWait();
 
@@ -133,6 +136,9 @@ class MEDIA_BLINK_EXPORT MultiBufferReader
   int64_t max_buffer_forward_;
   int64_t max_buffer_backward_;
 
+  // Currently pinned range.
+  Interval<MultiBuffer::BlockId> pinned_range_;
+
   // Current position in bytes.
   int64_t pos_;