Add support for cancellation of demuxer reads.

media/filters/blocking_url_protocol.cc

Index: media/filters/blocking_url_protocol.cc
diff --git a/media/filters/blocking_url_protocol.cc b/media/filters/blocking_url_protocol.cc
index c8340ee6385a0c722da4f78d21d603e1d37cfe89..9f8e95e35fd17a3796241bff98a455e394e78cc8 100644
--- a/media/filters/blocking_url_protocol.cc
+++ b/media/filters/blocking_url_protocol.cc
@@ -62,6 +62,9 @@ int BlockingUrlProtocol::Read(int size, uint8_t* data) {
     return AVERROR(EIO);
   }
 
+  if (last_read_bytes_ == DataSource::kAborted)
+    return AVERROR(EIO);
+
   read_position_ += last_read_bytes_;
   return last_read_bytes_;
 }