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

gpu/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/libfuzzer/fuzzer_test.gni")
 import("//testing/test.gni")
 import("//build/config/ui.gni")
 
 config("gpu_implementation") {
   defines = [ "GPU_IMPLEMENTATION" ]
@@ skipping 89 matching lines @@
     configs += [ "//build/config/gcc:rpath_for_built_shared_libraries" ]
   }
 }
 
 static_library("test_support") {
   testonly = true
   sources = [
     "command_buffer/client/gles2_interface_stub.cc",
     "command_buffer/client/gles2_interface_stub.h",
     "command_buffer/service/error_state_mock.cc",
+    "command_buffer/service/error_state_mock.h",
     "command_buffer/service/gles2_cmd_decoder_mock.cc",
+    "command_buffer/service/gles2_cmd_decoder_mock.h",
     "test_message_loop_type.h",
   ]
 
   public_deps = [
     ":gpu",
     "//gpu/command_buffer/client:gles2_interface",
   ]
   deps = [
     "//testing/gmock",
     "//testing/gtest",
@@ skipping 330 matching lines @@
       "//base/third_party/dynamic_annotations",
       "//gpu/command_buffer/common:gles2_utils",
       "//ui/gfx/geometry",
       "//ui/gl",
       "//ui/gl:test_support",
     ]
 
     libfuzzer_options = [ "max_len=16384" ]
   }
 }