DevTools: use --custom-devtools-frontend to specify bundle path

When built using debug_devtools=true, allow using --custom-devtools-frontend
command line flag to specify path to DevTools frontend resource files.

Bug: 988990
Change-Id: I90e84ba797397af77477838030961ebd6f2f8cac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1741813Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685538}

chrome/browser/ui/webui/devtools_ui.cc

@@ -369,7 +369,11 @@ void DevToolsDataSource::StartFileRequestForDebugDevtools(
     const std::string& path,
     const GotDataCallback& callback) {
   base::FilePath inspector_debug_dir;
-  if (!base::PathService::Get(chrome::DIR_INSPECTOR_DEBUG,
+  base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
+  if (command_line->HasSwitch(switches::kCustomDevtoolsFrontend)) {
+    inspector_debug_dir =
+        command_line->GetSwitchValuePath(switches::kCustomDevtoolsFrontend);
+  } else if (!base::PathService::Get(chrome::DIR_INSPECTOR_DEBUG,
                                      &inspector_debug_dir)) {
     callback.Run(CreateNotFoundResponse());
     return;