Fix "conversion of size_t to unsigned int" warning

net/dns/record_rdata.h

Index: net/dns/record_rdata.h
diff --git a/net/dns/record_rdata.h b/net/dns/record_rdata.h
index d2ca587c5e7811ebb72ab8fce6db6f1c854acc90..9e48b1c3162ee1cc5aed46995dd1a7bb509596f6 100644
--- a/net/dns/record_rdata.h
+++ b/net/dns/record_rdata.h
@@ -198,7 +198,7 @@ class NET_EXPORT_PRIVATE NsecRecordRdata : public RecordRdata {
   uint16_t Type() const override;
 
   // Length of the bitmap in bits.
-  unsigned bitmap_length() const { return bitmap_.size() * 8; }
+  uint16_t bitmap_length() const;
 
   // Returns bit i-th bit in the bitmap, where bits withing a byte are organized
   // most to least significant. If it is set, a record with rrtype i exists for