DevTools: Fix GN build flakiness

Source/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.
 
+blink_devtools_output_dir = "$root_gen_dir/blink/devtools/"
+
 gypi_values = exec_script(
     "//build/gypi_to_gn.py",
     [ rebase_path("devtools.gypi") ],
     "scope",
     [ "devtools.gypi" ])
 
 # Some of the files in the .gypi use GYP variable expansions, go through and
 # fix them.
 devtools_core_base_js_files = gypi_values.devtools_core_base_js_files - [
   "<@(devtools_main_js_files)",
@@ skipping 164 matching lines @@
 action("supported_css_properties") {
   script = "scripts/generate_supported_css.py"
 
   source_prereqs = [
     "../core/css/CSSPropertyNames.in",
     "../core/css/SVGCSSPropertyNames.in",
     "../core/css/CSSShorthands.in",
   ]
 
   outputs = [
-    "$root_gen_dir/blink/devtools/SupportedCSSProperties.js"
+    blink_devtools_output_dir + "SupportedCSSProperties.js"
   ]
 
   args = rebase_path(source_prereqs, root_build_dir) +
     rebase_path(outputs, root_build_dir)
 }
 
 action("frontend_protocol_sources") {
   script = "scripts/CodeGeneratorFrontend.py"
 
   source_prereqs = [ "protocol.json" ]
-  outputs = [ "$root_gen_dir/blink/devtools/InspectorBackendCommands.js" ]
+  outputs = [ blink_devtools_output_dir + "InspectorBackendCommands.js" ]
 
   args = rebase_path(source_prereqs, root_build_dir) + [
-    "--output_js_dir", rebase_path("$root_gen_dir/blink", root_build_dir),
+    "--output_js_dir", rebase_path(blink_devtools_output_dir, root_build_dir),
   ]
 }
 
 # Runs the inline_js_imports script.
 #  input: (String) The input .js file to read.
 #  imported_files: (List of strings) The list of files the input depends on.
 #  output: (String) The .js file to write.
 template("inline_js_imports") {
   assert(defined(invoker.input), target_name)
   assert(defined(invoker.imported_files), target_name)
@@ skipping 16 matching lines @@
   input = "front_end/audits/AuditsPanel.js"
   imported_files = gypi_values.devtools_audits_js_files
   output = "$root_out_dir/resources/inspector/audits/AuditsPanel.js"
 }
 
 action("build_core_module") {
   script = "scripts/concatenate_js_files.py"
   input_page = "front_end/inspector.html"
   source_prereqs = [
     input_page,
-    "$root_gen_dir/blink/devtools/InspectorBackendCommands.js",
-    "$root_gen_dir/blink/devtools/SupportedCSSProperties.js",
-    "$root_gen_dir/blink/devtools/common/modules.js",
+    blink_devtools_output_dir + "InspectorBackendCommands.js",
+    blink_devtools_output_dir + "SupportedCSSProperties.js",
+    blink_devtools_output_dir + "common/modules.js",
   ] + devtools_core_js_files
 
   search_path = [
-    "$root_gen_dir/blink",
+    blink_devtools_output_dir,
     "front_end",
   ]
   outputs = [
     "$root_out_dir/resources/inspector/main/Main.js",
   ]
 
   args = [
     rebase_path(input_page, root_build_dir),
   ]
   args += rebase_path(search_path, root_build_dir)
@@ skipping 102 matching lines @@
   input = "front_end/temp_storage_shared_worker/TempStorageSharedWorker.js"
   imported_files = gypi_values.devtools_temp_storage_shared_worker_js_files
   output = "$root_out_dir/resources/inspector/temp_storage_shared_worker/TempStorageSharedWorker.js"
 }
 
 action("concatenated_module_descriptors") {
   script = "scripts/concatenate_module_descriptors.py"
 
   input = "front_end/common/modules.js"
   source_prereqs = [ input ] + gypi_values.devtools_module_json_files
-  outputs = [ "$root_gen_dir/blink/devtools/common/modules.js" ]
+  outputs = [ blink_devtools_output_dir + "common/modules.js" ]
 
   args = rebase_path([ input ] + outputs + gypi_values.devtools_module_json_files, root_build_dir)
 }
 
 action("concatenated_devtools_css") {
   script = "scripts/concatenate_css_files.py"
 
   input_page = "front_end/inspector.html"
   source_prereqs = [ input_page ] + all_devtools_files
   search_path = [ "front_end" ]
 
   outputs = [ "$root_out_dir/resources/inspector/inspector.css" ]
 
   args = [ rebase_path(input_page, root_build_dir) ] +
     rebase_path(search_path, root_build_dir) +
     rebase_path(outputs, root_build_dir)
 }