[ExploreSites][MostLikely]: Add new tile source for explore sites.

This will support a stronger integration between the Explore Sites
buttons and the Most Likely tile framework.

Bug: 966024
Change-Id: I6b7e7f577b52f6fd40748a211df13eab3f6ae66c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1623069
Reviewed-by: Marc Treib <[email protected]>
Reviewed-by: Mark Pearson <[email protected]>
Commit-Queue: Justin DeWitt <[email protected]>
Cr-Commit-Position: refs/heads/master@{#662400}
diff --git a/components/ntp_tiles/metrics.cc b/components/ntp_tiles/metrics.cc
index 2edf447b..a942354 100644
--- a/components/ntp_tiles/metrics.cc
+++ b/components/ntp_tiles/metrics.cc
@@ -27,6 +27,7 @@
 const char kHistogramWhitelistName[] = "whitelist";
 const char kHistogramHomepageName[] = "homepage";
 const char kHistogramCustomLinksName[] = "custom_links";
+const char kHistogramExploreName[] = "explore";
 
 // Suffixes for the various icon types.
 const char kTileTypeSuffixIconColor[] = "IconsColor";
@@ -57,6 +58,8 @@
       return kHistogramHomepageName;
     case TileSource::CUSTOM_LINKS:
       return kHistogramCustomLinksName;
+    case TileSource::EXPLORE:
+      return kHistogramExploreName;
   }
   NOTREACHED();
   return std::string();