Convert startup_metric_utils to use mojo.

components/startup_metric_utils/common/BUILD.gn

Index: components/startup_metric_utils/common/BUILD.gn
diff --git a/components/startup_metric_utils/common/BUILD.gn b/components/startup_metric_utils/common/BUILD.gn
index d9b2f0996e79ea5fee4a73ed06d09b241d90085a..54fd89587c0beed768a2c28a7b67cba9c7dfab28 100644
--- a/components/startup_metric_utils/common/BUILD.gn
+++ b/components/startup_metric_utils/common/BUILD.gn
@@ -2,13 +2,12 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//mojo/public/tools/bindings/mojom.gni")
+
 source_set("common") {
   sources = [
     "pre_read_field_trial_utils_win.cc",
     "pre_read_field_trial_utils_win.h",
-    "startup_metric_message_generator.cc",
-    "startup_metric_message_generator.h",
-    "startup_metric_messages.h",
   ]
 
   deps = [
@@ -17,3 +16,12 @@ source_set("common") {
     "//ipc",
   ]
 }
+
+mojom("interfaces") {
+  sources = [
+    "startup_metric.mojom",
+  ]
+  deps = [
+    "//mojo/common:common_custom_types",
+  ]
+}