Add missing .h files to .gn files that have a .cc file next to it in the gn file already.

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.
 
 import("//testing/test.gni")
 import("//third_party/protobuf/proto_library.gni")
 
 component("tracing") {
   sources = [
     "child/child_trace_message_filter.cc",
@@ skipping 81 matching lines @@
   sources = [
     "child/child_trace_message_filter_unittest.cc",
     "common/graphics_memory_dump_provider_android_unittest.cc",
     "common/process_metrics_memory_dump_provider_unittest.cc",
     "core/proto_utils_unittest.cc",
     "core/proto_zero_message_unittest.cc",
     "core/scattered_stream_writer_unittest.cc",
     "core/trace_buffer_writer_unittest.cc",
     "core/trace_ring_buffer_unittest.cc",
     "test/fake_scattered_buffer.cc",
+    "test/fake_scattered_buffer.h",
     "test/proto_zero_generation_unittest.cc",
   ]
 
   deps = [
     ":proto_zero_testing_messages",
     ":tracing",
     "//base/test:test_support",
     "//components/tracing/proto:golden_protos_for_tests",
     "//ipc",
     "//testing/gmock:gmock",
@@ skipping 25 matching lines @@
     "//testing/scripts/common.py",
     "//testing/xvfb.py",
     "//testing/scripts/run_gtest_perf_test.py",
     "//tools/perf/generate_legacy_perf_dashboard_json.py",
   ]
 
   if (is_android) {
     deps += [ "//testing/android/native_test:native_test_native_code" ]
   }
 }