Handle the "debug_devtools" build arg in GN builds

chrome/browser/ui/BUILD.gn

Index: chrome/browser/ui/BUILD.gn
diff --git a/chrome/browser/ui/BUILD.gn b/chrome/browser/ui/BUILD.gn
index dbd8d43d1d31b696c8c620171396ff9c04fb509a..4c0dca57f9ad6bf6334221dc743dafec6f00cc33 100644
--- a/chrome/browser/ui/BUILD.gn
+++ b/chrome/browser/ui/BUILD.gn
@@ -19,7 +19,10 @@ static_library("ui") {
   defines = []
   libs = []
 
-  configs += [ "//build/config/compiler:wexit_time_destructors" ]
+  configs += [
+    "//build/config/compiler:wexit_time_destructors",
+    "//third_party/WebKit/public:debug_devtools",
+  ]
 
   # Since browser and browser_ui actually depend on each other,
   # we must omit the dependency from browser_ui to browser.
@@ -142,9 +145,6 @@ static_library("ui") {
       #"//native_client/src/trusted/service_runtime/service_runtime.gyp:sel",  TODO(GYP)
     ]
   }
-  if (debug_devtools) {
-    defines += [ "DEBUG_DEVTOOLS=1" ]
-  }
   if (enable_configuration_policy) {
     sources += rebase_path(gypi_values.chrome_browser_ui_policy_sources,
                            ".", "//chrome")