[Sync] Initial implementation of foreign sessions suggestions provider.

components/ntp_snippets/content_suggestions_metrics.cc

Index: components/ntp_snippets/content_suggestions_metrics.cc
diff --git a/components/ntp_snippets/content_suggestions_metrics.cc b/components/ntp_snippets/content_suggestions_metrics.cc
index b83522a73a3d6a7768dbff941e392f829b0d8eac..2876eac92cce11a637ba5c72224fec193cff2a6c 100644
--- a/components/ntp_snippets/content_suggestions_metrics.cc
+++ b/components/ntp_snippets/content_suggestions_metrics.cc
@@ -45,6 +45,8 @@ const char kHistogramMoreButtonClicked[] =
 
 const char kPerCategoryHistogramFormat[] = "%s.%s";
 
+// Each suffix here should correspond to an entry under histogram suffix
+// ContentSuggestionCategory in histograms.xml.
 std::string GetCategorySuffix(Category category) {
   static_assert(
       std::is_same<decltype(category.id()), typename base::underlying_type<
@@ -64,6 +66,8 @@ std::string GetCategorySuffix(Category category) {
       return "Bookmarks";
     case KnownCategories::PHYSICAL_WEB_PAGES:
       return "PhysicalWeb";
+    case KnownCategories::FOREIGN_TABS:
+      return "ForeignTabs";
     case KnownCategories::ARTICLES:
       return "Articles";
     case KnownCategories::LOCAL_CATEGORIES_COUNT: