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

content/test/BUILD.gn

Index: content/test/BUILD.gn
diff --git a/content/test/BUILD.gn b/content/test/BUILD.gn
index c3f0ecc13d1badf81e0ca84bbb317e7087912ee5..c535971a67c4cecdbf87b72b70d1232b3b1ccc4f 100644
--- a/content/test/BUILD.gn
+++ b/content/test/BUILD.gn
@@ -928,7 +928,9 @@ test("content_browsertests") {
   if (enable_plugins) {
     sources += [
       "../renderer/pepper/fake_pepper_plugin_instance.cc",
+      "../renderer/pepper/fake_pepper_plugin_instance.h",
       "../renderer/pepper/mock_renderer_ppapi_host.cc",
+      "../renderer/pepper/mock_renderer_ppapi_host.h",
       "../renderer/pepper/pepper_device_enumeration_host_helper_unittest.cc",
       "../renderer/pepper/pepper_file_chooser_host_unittest.cc",
       "../renderer/pepper/pepper_graphics_2d_host_unittest.cc",
@@ -1619,6 +1621,7 @@ test("content_unittests") {
       "../renderer/media/media_stream_video_source_unittest.cc",
       "../renderer/media/media_stream_video_track_unittest.cc",
       "../renderer/media/mock_constraint_factory.cc",
+      "../renderer/media/mock_constraint_factory.h",
       "../renderer/media/mock_media_stream_registry.cc",
       "../renderer/media/mock_media_stream_registry.h",
       "../renderer/media/mock_media_stream_video_sink.cc",