[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 5d4fbe169ad5a462da482bcc432b02bbbaa3b785..dfe86a9e658720dbc4998e082945e81378aefcc6 100644
--- a/components/ntp_snippets/content_suggestions_metrics.cc
+++ b/components/ntp_snippets/content_suggestions_metrics.cc
@@ -43,6 +43,8 @@ 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) {
   // TODO(treib): Find a way to produce a compile error if a known category
   // isn't listed here.
@@ -52,6 +54,8 @@ std::string GetCategorySuffix(Category category) {
     return "Downloads";
   if (category.IsKnownCategory(KnownCategories::BOOKMARKS))
     return "Bookmarks";
+  if (category.IsKnownCategory(KnownCategories::FOREIGN_TABS))
+    return "ForeignTabs";
   if (category.IsKnownCategory(KnownCategories::ARTICLES))
     return "Articles";
   // All other categories go into a single "Experimental" bucket.