Handle the "debug_devtools" build arg in GN builds

chrome/browser/BUILD.gn

Index: chrome/browser/BUILD.gn
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index 8181e41c37ef4f86bd5da4905789310b9cc538ab..5df54bef6b18c2396d1d57b72b59ac768d07f0b4 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -148,6 +148,10 @@ static_library("browser") {
 
     defines = nacl_defines
 
+    if (debug_devtools) {
+      defines += [ "DEBUG_DEVTOOLS=1" ]
+    }
+
     deps += [
       "//apps",
       "//cc",
@@ -273,10 +277,7 @@ static_library("browser") {
                            ".", "//chrome")
     #deps += [ "//components/nacl:nacl_browser" ]  TODO(GYP)
   }
-  # ["debug_devtools==1", {  TODO(GYP)
-  #   "defines": [
-  #     "DEBUG_DEVTOOLS=1",
-  # }],
+
   if (enable_configuration_policy) {
     sources += rebase_path(
         gypi_values.chrome_browser_policy_shared_with_ios_sources,