Allow startup with missing V8 snapshot file.

ash/shell/content_client/shell_content_browser_client.h

Index: ash/shell/content_client/shell_content_browser_client.h
diff --git a/ash/shell/content_client/shell_content_browser_client.h b/ash/shell/content_client/shell_content_browser_client.h
index 9ee3373f907959048a81e2645535dd70abb25d49..a5a99f8709fdbd1406585aa5ba4e62fb46af3877 100644
--- a/ash/shell/content_client/shell_content_browser_client.h
+++ b/ash/shell/content_client/shell_content_browser_client.h
@@ -33,8 +33,8 @@ class ShellContentBrowserClient : public content::ContentBrowserClient {
       content::BrowserContext* browser_context,
       content::ProtocolHandlerMap* protocol_handlers,
       content::URLRequestInterceptorScopedVector request_interceptors) override;
-  void AppendExtraCommandLineSwitches(base::CommandLine* command_line,
-                                      int child_process_id) override;
+  void AppendMappedFileCommandLineSwitches(
+      base::CommandLine* command_line) override;
 #if defined(OS_POSIX) && !defined(OS_MACOSX)
   void GetAdditionalMappedFilesForChildProcess(
       const base::CommandLine& command_line,
@@ -45,6 +45,9 @@ class ShellContentBrowserClient : public content::ContentBrowserClient {
   content::ShellBrowserContext* browser_context();
 
  private:
+  bool natives_fd_exists() { return v8_natives_fd_.is_valid(); }
+  bool snapshot_fd_exists() { return v8_snapshot_fd_.is_valid(); }
+
 #if defined(OS_POSIX) && !defined(OS_MACOSX)
   base::ScopedFD v8_natives_fd_;
   base::ScopedFD v8_snapshot_fd_;