Extended the ProtoDatabase to provide LoadKeys() functionality.

components/ntp_snippets/remote/ntp_snippets_database.cc

Index: components/ntp_snippets/remote/ntp_snippets_database.cc
diff --git a/components/ntp_snippets/remote/ntp_snippets_database.cc b/components/ntp_snippets/remote/ntp_snippets_database.cc
index 5ded341343d74ca9813d4f1ead438928cdd52e88..41498820e306410985565ca27354dae0afda5cc7 100644
--- a/components/ntp_snippets/remote/ntp_snippets_database.cc
+++ b/components/ntp_snippets/remote/ntp_snippets_database.cc
@@ -133,6 +133,32 @@ void NTPSnippetsDatabase::DeleteImages(const NTPSnippet::PtrVector& snippets) {
   DeleteImagesImpl(std::move(keys_to_remove));
 }
 
+void NTPSnippetsDatabase::DeleteUnreferencedImages(
+    std::unique_ptr<std::set<std::string>> references,
+    bool success,
+    std::unique_ptr<std::vector<std::string>> keys) {
+  if (!success) {
+    DVLOG(1) << "NTPSnippetsDatabase garbage collection failed.";
+    OnDatabaseError();
+    return;
+  }
+  auto keys_to_remove = base::MakeUnique<std::vector<std::string>>();
+  for (const std::string& key : *keys) {
+    if (references->count(key) == 0) {
+      keys_to_remove->emplace_back(key);
+    }
+  }
+  DeleteImagesImpl(std::move(keys_to_remove));
+}
+
+void NTPSnippetsDatabase::GarbageCollectImages(
+    std::unique_ptr<std::set<std::string>> alive_snippets) {
+  DCHECK(image_database_initialized_);
+  image_database_->LoadKeys(base::Bind(
+      &NTPSnippetsDatabase::DeleteUnreferencedImages,
+      weak_ptr_factory_.GetWeakPtr(), base::Passed(std::move(alive_snippets))));
+}
+
 void NTPSnippetsDatabase::OnDatabaseInited(bool success) {
   DCHECK(!database_initialized_);
   if (!success) {