Adds a UKM debug page at chrome://ukm

BUG=718674

Review-Url: https://codereview.chromium.org/2858683006
Cr-Commit-Position: refs/heads/master@{#471084}
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index bf646f6f..3aab0f0 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -1582,6 +1582,7 @@
     "//components/translate/core/browser",
     "//components/translate/core/common",
     "//components/ukm:observers",
+    "//components/ukm/debug_page",
     "//components/undo",
     "//components/update_client",
     "//components/upload_list",