ntp_tiles::metrics: Add rappor metrics for impression URLs per icon type.

components/ntp_tiles/BUILD.gn

Index: components/ntp_tiles/BUILD.gn
diff --git a/components/ntp_tiles/BUILD.gn b/components/ntp_tiles/BUILD.gn
index bd64210365e0d4189eeb059f384b4a0b7abed249..457e7745aeafb1700dbfd5aec4de10efff0d59fd 100644
--- a/components/ntp_tiles/BUILD.gn
+++ b/components/ntp_tiles/BUILD.gn
@@ -46,6 +46,7 @@ static_library("ntp_tiles") {
     "//components/image_fetcher",
     "//components/pref_registry",
     "//components/prefs",
+    "//components/rappor/public",
     "//components/search_engines",
     "//components/url_formatter",
     "//components/variations",
@@ -77,6 +78,7 @@ source_set("unit_tests") {
   testonly = true
   sources = [
     "icon_cacher_unittest.cc",
+    "metrics_unittest.cc",
     "most_visited_sites_unittest.cc",
     "popular_sites_unittest.cc",
   ]
@@ -89,6 +91,7 @@ source_set("unit_tests") {
     "//components/favicon_base",
     "//components/image_fetcher",
     "//components/pref_registry:pref_registry",
+    "//components/rappor:test_support",
     "//components/sync_preferences:test_support",
     "//net:test_support",
     "//testing/gmock",