Handle the "debug_devtools" build arg in GN builds

chrome/browser/devtools/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.
 
 action("devtools_protocol_constants") {
   script = "//content/public/browser/devtools_protocol_constants_generator.py"
 
   blink_protocol = "//third_party/WebKit/Source/devtools/protocol.json"
   browser_protocol = "//content/browser/devtools/browser_protocol.json"
   inputs = [ blink_protocol, browser_protocol ]
@@ skipping 22 matching lines @@
     "devtools_network_protocol_handler.h",
     "devtools_network_transaction.cc",
     "devtools_network_transaction.h",
     "devtools_network_transaction_factory.cc",
     "devtools_network_transaction_factory.h",
     "devtools_protocol.cc",
     "devtools_protocol.h"
   ]
   sources += get_target_outputs(":devtools_protocol_constants")
 
-  # TODO(GYP)
-  #     ['debug_devtools==1', {
-  #       'defines': [
-  #         'DEBUG_DEVTOOLS=1',
-  #        ],
-  #     }],
   if (is_win) {
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     cflags = [ "/wd4267" ]
   }
-  configs += [ "//build/config/compiler:wexit_time_destructors" ]
+  configs += [
+    "//build/config/compiler:wexit_time_destructors",
+    "//third_party/WebKit/public:debug_devtools",
+  ]
 
   deps = [
     ":devtools_protocol_constants",
     "//base",
     "//content/public/browser",
     "//net",
   ]
 
   if (!is_android) {
     deps += [
@@ skipping 53 matching lines @@
       "devtools_toggle_action.h",
       "devtools_ui_bindings.cc",
       "devtools_ui_bindings.h",
       "devtools_window.cc",
       "devtools_window.h",
       "remote_debugging_server.cc",
       "remote_debugging_server.h"
     ]
   }
 }