Add UI_DEVTOOLS_EXPORT so tests can mock FrontendChannel

components/ui_devtools/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.
 
 _inspector_protocol = "//third_party/inspector_protocol"
 import("$_inspector_protocol/inspector_protocol.gni")
 
 _protocol_generated = [
   "DOM.cpp",
   "DOM.h",
@@ skipping 38 matching lines @@
 
   outputs = _protocol_generated
 }
 
 source_set("ui_devtools") {
   sources = rebase_path(_protocol_generated, ".", target_gen_dir)
   sources += [
     "devtools_base_agent.h",
     "devtools_client.cc",
     "devtools_client.h",
+    "devtools_export.h",
     "devtools_server.cc",
     "devtools_server.h",
     "string_util.cc",
     "string_util.h",
     "switches.cc",
     "switches.h",
   ]
 
+  defines = [ "UI_DEVTOOLS_IMPLEMENTATION" ]
+
   cflags = []
   if (is_win) {
     cflags += [ "/wd4800" ]  # Value forced to bool.
   }
 
   deps = [
     ":protocol_generated_sources",
     "//base",
     "//net",
     "//net:http_server",
   ]
 }