Start work on GN build for content/common

components/tracing/BUILD.gn

Index: components/tracing/BUILD.gn
diff --git a/components/metrics/BUILD.gn b/components/tracing/BUILD.gn
similarity index 56%
copy from components/metrics/BUILD.gn
copy to components/tracing/BUILD.gn
index 72fc0fe1227ecd9d8d0ed56106944998391a40e6..c2e05190ad6e7a0996e2713071e35a4bea4525fe 100644
--- a/components/metrics/BUILD.gn
+++ b/components/tracing/BUILD.gn
@@ -2,13 +2,16 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-source_set("metrics") {
+source_set("tracing") {
   sources = [
-    "metrics_hashes.cc",
-    "metrics_hashes.h",
+    "child_trace_message_filter.cc",
+    "child_trace_message_filter.h",
+    "tracing_messages.cc",
+    "tracing_messages.h",
   ]
 
   deps = [
     "//base",
+    "//ipc",
   ]
 }