Reland again [Startup Tracing] Add --trace-config-file flag

components/tracing/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 component("tracing") {
   sources = [
     "child_memory_dump_manager_delegate_impl.cc",
     "child_memory_dump_manager_delegate_impl.h",
     "child_trace_message_filter.cc",
     "child_trace_message_filter.h",
     "tracing_export.h",
     "tracing_messages.cc",
     "tracing_messages.h",
   ]
 
   defines = [ "TRACING_IMPLEMENTATION" ]
 
   deps = [
     "//base",
     "//ipc",
   ]
 }
 
 component("startup_tracing") {
   sources = [
-    "startup_tracing.cc",
-    "startup_tracing.h",
+    "trace_config_file.cc",
+    "trace_config_file.h",
     "tracing_export.h",
     "tracing_switches.cc",
     "tracing_switches.h",
   ]
 
   defines = [ "TRACING_IMPLEMENTATION" ]
 
   deps = [
     "//base",
   ]
 }
+
+source_set("unit_tests") {
+  testonly = true
+
+  sources = [
+    "trace_config_file_unittest.cc",
+  ]
+
+  deps = [
+    ":startup_tracing",
+    "//base/test:test_support",
+    "//testing/gtest",
+  ]
+}