Add missing test file to tracing:unit_tests

components/tracing/BUILD.gn

Index: components/tracing/BUILD.gn
diff --git a/components/tracing/BUILD.gn b/components/tracing/BUILD.gn
index 927aeb67894b1e398c5a2f96a463126df2fd9a95..e5200dd9c934009f2b3ce06066eafec8ade451d4 100644
--- a/components/tracing/BUILD.gn
+++ b/components/tracing/BUILD.gn
@@ -51,6 +51,7 @@ source_set("unit_tests") {
   testonly = true
 
   sources = [
+    "child_trace_message_filter_unittest.cc",

[Primiano Tucci (use gerrit), 2016/05/19 17:06:33]

FYI in gyp they seem excluded for IOS.
But I guess gn for IOS is not a thing yet and somebody else will have to deal
with that?

     "graphics_memory_dump_provider_android_unittest.cc",
     "process_metrics_memory_dump_provider_unittest.cc",
   ]
@@ -58,6 +59,8 @@ source_set("unit_tests") {
   deps = [
     ":tracing",
     "//base/test:test_support",
+    "//ipc",
+    "//testing/gmock:gmock",
     "//testing/gtest",
   ]