Fix Mac build after https://codereview.chromium.org/1407643004

snapshot/mac/process_snapshot_mac.cc

Index: snapshot/mac/process_snapshot_mac.cc
diff --git a/snapshot/mac/process_snapshot_mac.cc b/snapshot/mac/process_snapshot_mac.cc
index 215536cc196d036f6db690ff970d1629dc410549..b37123c8235b6c5220056d4b3cd85a946ea2af6e 100644
--- a/snapshot/mac/process_snapshot_mac.cc
+++ b/snapshot/mac/process_snapshot_mac.cc
@@ -192,6 +192,11 @@ std::vector<const MemoryMapRegionSnapshot*> ProcessSnapshotMac::MemoryMap()
   return std::vector<const MemoryMapRegionSnapshot*>();
 }
 
+std::vector<HandleSnapshot> ProcessSnapshotMac::Handles() const {
+  INITIALIZATION_STATE_DCHECK_VALID(initialized_);
+  return std::vector<HandleSnapshot>();
+}
+
 std::vector<const MemorySnapshot*> ProcessSnapshotMac::ExtraMemory() const {
   INITIALIZATION_STATE_DCHECK_VALID(initialized_);
   return std::vector<const MemorySnapshot*>();