Use base::flat_set<> rather than std::set<> for ResourceType set

components/safe_browsing_db/remote_database_manager.cc

Index: components/safe_browsing_db/remote_database_manager.cc
diff --git a/components/safe_browsing_db/remote_database_manager.cc b/components/safe_browsing_db/remote_database_manager.cc
index 2def386d5f8b885838ebcc49de2cc136c07e5532..3c4ec552c46b33fbbcdff8e8c80ef198b06e4136 100644
--- a/components/safe_browsing_db/remote_database_manager.cc
+++ b/components/safe_browsing_db/remote_database_manager.cc
@@ -99,9 +99,12 @@ void RemoteSafeBrowsingDatabaseManager::ClientRequest::OnRequestDone(
 
 // TODO(nparker): Add more tests for this class
 RemoteSafeBrowsingDatabaseManager::RemoteSafeBrowsingDatabaseManager() {
+  // Adding one element at a time to a base::flat_set is inefficient. Creating
+  // the list of elements in a vector first avoids extra allocations.
+  std::vector<content::ResourceType> types_to_check;

[Nathan Parker, 2017/04/25 15:23:14]

This seems seems like overkill for 18 elements that are added once at startup. 
Could we simplify the code by adding it directly to the set?  Or sizing the set
a head of time?

[Adam Rice, 2017/04/26 05:18:27]

I've tried adding directly to the set in patch set 2. I don't like adding
overhead during startup, but the simplification is compelling.

By my back-of-the-envelope calculation it roughly doubles the number of
operations needed for a full set with all 18 items. It's still an order of
magnitude cheaper to initialise that the old std::set.

   // Decide which resource types to check. These two are the minimum.
-  resource_types_to_check_.insert(content::RESOURCE_TYPE_MAIN_FRAME);
-  resource_types_to_check_.insert(content::RESOURCE_TYPE_SUB_FRAME);
+  types_to_check.push_back(content::RESOURCE_TYPE_MAIN_FRAME);
+  types_to_check.push_back(content::RESOURCE_TYPE_SUB_FRAME);
 
   // The param is expected to be a comma-separated list of ints
   // corresponding to the enum types.  We're keeping this finch
@@ -122,7 +125,7 @@ RemoteSafeBrowsingDatabaseManager::RemoteSafeBrowsingDatabaseManager() {
         case content::RESOURCE_TYPE_FAVICON:
           break;
         default:
-          resource_types_to_check_.insert(t);
+          types_to_check.push_back(t);
       }
     }
   } else {
@@ -132,10 +135,13 @@ RemoteSafeBrowsingDatabaseManager::RemoteSafeBrowsingDatabaseManager() {
       int i;
       if (base::StringToInt(val_str, &i) && i >= 0 &&
           i < content::RESOURCE_TYPE_LAST_TYPE) {
-        resource_types_to_check_.insert(static_cast<content::ResourceType>(i));
+        types_to_check.push_back(static_cast<content::ResourceType>(i));
       }
     }
   }
+  base::flat_set<content::ResourceType> resource_types_to_check_temporary(
+      std::move(types_to_check), base::KEEP_FIRST_OF_DUPES);
+  resource_types_to_check_.swap(resource_types_to_check_temporary);
 }
 
 RemoteSafeBrowsingDatabaseManager::~RemoteSafeBrowsingDatabaseManager() {