Add missing header files to GN files (3 of N)

content/test/fuzzer/BUILD.gn

 # Copyright 2016 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.
 
 # Fuzzers for content/ components.
 
 import("//testing/libfuzzer/fuzzer_test.gni")
 import("//third_party/protobuf/proto_library.gni")
 
 # Empty group for package discovery.
 group("fuzzer") {
 }
 
 # gn complains of private dependencies in component build.
 if (!is_component_build) {
   source_set("fuzzer_support") {
     sources = [
       "fuzzer_support.cc",
+      "fuzzer_support.h",
     ]
     public_deps = [
       "//base",
       "//content/renderer:for_content_tests",
       "//content/shell:content_shell_lib",
       "//content/test:test_support",
       "//gin",
     ]
     testonly = true
   }
@@ skipping 51 matching lines @@
     ]
   }
 
   proto_library("html_tree_proto") {
     sources = [
       "html_tree.proto",
     ]
     testonly = true
   }
 }