Disable etw_manifest in the GN Windows build.

build/win/message_compiler.gni

 # Copyright 2015 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.
 
 assert(is_win, "This only runs on Windows.")
 
 # Runs mc.exe over a list of sources. The outputs (a header and rc file) are
 # placed in the target gen dir, and compiled.
 #
 # sources
 #   List of message files to process.
 #
 # user_mode_logging (optional bool)
 #   Generates user-mode logging code. Defaults to false (no logging code).
 #
+# compile_generated_code (optional, deafults = true)
+#   If unset or true, the generated code will be compiled and linked into
+#   targets that depend on it. If set to false, the .h and .rc files will only
+#   be generated.
+#
 # deps, public_deps, visibility
 #   Normal meaning.
 template("message_compiler") {
-  action_name = "${target_name}_mc"
-  source_set_name = target_name
+  if (defined(invoker.compile_generated_code) &&
+      !invoker.compile_generated_code) {
+    compile_generated_code = false
+    action_name = target_name
+  } else {
+    compile_generated_code = true
+    action_name = "${target_name}_mc"
+    source_set_name = target_name
+  }
 
   action_foreach(action_name) {
-    visibility = [ ":$source_set_name" ]
+    if (compile_generated_code) {
+      visibility = [ ":$source_set_name" ]
+    } else {
+      forward_variables_from(invoker, [ "visibility" ])
+    }
 
     script = "//build/win/message_compiler.py"
 
     outputs = [
       "$target_gen_dir/{{source_name_part}}.h",
       "$target_gen_dir/{{source_name_part}}.rc",
     ]
 
     args = [
       # The first argument is the environment file saved to the build
@@ skipping 19 matching lines @@
     args += [ "{{source}}" ]
 
     forward_variables_from(invoker,
                            [
                              "deps",
                              "public_deps",
                              "sources",
                            ])
   }
 
-  # Compile the generated rc file.
-  source_set(source_set_name) {
-    forward_variables_from(invoker, [ "visibility" ])
-    sources = get_target_outputs(":$action_name")
-    deps = [
-      ":$action_name",
-    ]
+  if (compile_generated_code) {
+    # Compile the generated rc file.
+    source_set(source_set_name) {
+      forward_variables_from(invoker, [ "visibility" ])
+      sources = get_target_outputs(":$action_name")
+      deps = [
+        ":$action_name",
+      ]
+    }
   }
 }