Switch Safe Browsing's metadata from string to struct.

components/safe_browsing_db/util.cc

Index: components/safe_browsing_db/util.cc
diff --git a/components/safe_browsing_db/util.cc b/components/safe_browsing_db/util.cc
index 18dd9f88daebe45d854ea8fc152f0328c23dab86..153da706dc0d3d35d2a9f3ee688f86ce0560e88d 100644
--- a/components/safe_browsing_db/util.cc
+++ b/components/safe_browsing_db/util.cc
@@ -29,6 +29,12 @@ bool IsKnownList(const std::string& name) {
 }
 }  // namespace
 
+// ThreatMetadata ------------------------------------------------------------
+ThreatMetadata::ThreatMetadata()
+    : threat_pattern_type(ThreatPatternType::NONE) {}
+
+ThreatMetadata::~ThreatMetadata() {}
+
 // SBCachedFullHashResult ------------------------------------------------------
 
 SBCachedFullHashResult::SBCachedFullHashResult() {}