printing: Use MD5DigestToBase16() to get a human-readable hexadecimal string.

printing/image.cc

Index: printing/image.cc
diff --git a/printing/image.cc b/printing/image.cc
index 5c832ad9d6e3fabab8ca558ff92fd9dd52b564d2..cb4776978f01f9fbfbf51434d271a8114ecda2d9 100644
--- a/printing/image.cc
+++ b/printing/image.cc
@@ -55,7 +55,7 @@ Image::~Image() {}
 std::string Image::checksum() const {
   base::MD5Digest digest;
   base::MD5Sum(&data_[0], data_.size(), &digest);
-  return base::HexEncode(&digest, sizeof(digest));
+  return base::MD5DigestToBase16(digest);
 }
 
 bool Image::SaveToPng(const base::FilePath& filepath) const {