Extended the ProtoDatabase to provide LoadKeys() functionality.

components/ntp_snippets/remote/ntp_snippets_service.cc

Index: components/ntp_snippets/remote/ntp_snippets_service.cc
diff --git a/components/ntp_snippets/remote/ntp_snippets_service.cc b/components/ntp_snippets/remote/ntp_snippets_service.cc
index 64eb84f7ad6f0f60080ad4e35302c5c8ae904df7..4acda602c6cb089bbd7c340b2b7910bd0ea41340 100644
--- a/components/ntp_snippets/remote/ntp_snippets_service.cc
+++ b/components/ntp_snippets/remote/ntp_snippets_service.cc
@@ -12,6 +12,7 @@
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/location.h"
+#include "base/memory/ptr_util.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/metrics/sparse_histogram.h"
 #include "base/path_service.h"
@@ -490,6 +491,9 @@ void NTPSnippetsService::OnDatabaseLoaded(NTPSnippet::PtrVector snippets) {
   DCHECK_EQ(1u, categories_.size());  // Only articles category, so far.
   DCHECK(categories_.find(articles_category_) != categories_.end());
 
+  ClearExpiredDismissedSnippets();
+  ClearOrphanedImages(snippets);
+
   // TODO(sfiera): support non-article categories in database.
   CategoryContent* content = &categories_[articles_category_];
   for (std::unique_ptr<NTPSnippet>& snippet : snippets) {
@@ -505,8 +509,6 @@ void NTPSnippetsService::OnDatabaseLoaded(NTPSnippet::PtrVector snippets) {
               return lhs->score() > rhs->score();
             });
 
-  ClearExpiredDismissedSnippets();
-  ClearOrphanedImages();
   FinishInitialization();
 }
 
@@ -591,7 +593,6 @@ void NTPSnippetsService::OnFetchFinished(
         UMA_HISTOGRAM_SPARSE_SLOWLY("NewTabPage.Snippets.NumArticlesFetched",
                                     fetched_category.snippets.size());
       }
-
       ReplaceSnippets(category, std::move(fetched_category.snippets));
     }
   }
@@ -767,8 +768,13 @@ void NTPSnippetsService::ClearExpiredDismissedSnippets() {
   }
 }
 
-void NTPSnippetsService::ClearOrphanedImages() {
-  // TODO(jkrcal): Implement. crbug.com/649009
+void NTPSnippetsService::ClearOrphanedImages(
+    const NTPSnippet::PtrVector& snippets) {
+  auto alive_snippets = base::MakeUnique<std::set<std::string>>();
+  for (const auto& snippet_ptr : snippets) {
+    alive_snippets->insert(snippet_ptr->id());
+  }
+  database_->GarbageCollectImages(std::move(alive_snippets));
 }
 
 void NTPSnippetsService::NukeAllSnippets() {