Add support to extension_shell and ash_shell to use external V8 snapshot files.

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 1a5eb4403af7b0f771014d0c682f9bd02f0369cf..df62338d8e4cc013fc01d76b501d884e1e69154b 100644
--- a/ash/shell/content_client/shell_content_browser_client.h
+++ b/ash/shell/content_client/shell_content_browser_client.h
@@ -33,10 +33,21 @@ class ShellContentBrowserClient : public content::ContentBrowserClient {
       content::BrowserContext* browser_context,
       content::ProtocolHandlerMap* protocol_handlers,
       content::URLRequestInterceptorScopedVector request_interceptors) override;
+#if defined(OS_POSIX) && !defined(OS_MACOSX)
+  void GetAdditionalMappedFilesForChildProcess(
+      const base::CommandLine& command_line,
+      int child_process_id,
+      content::FileDescriptorInfo* mappings) override;
+#endif
 
   content::ShellBrowserContext* browser_context();
 
  private:
+#if defined(OS_POSIX) && !defined(OS_MACOSX)
+  base::ScopedFD v8_natives_fd_;
+  base::ScopedFD v8_snapshot_fd_;
+#endif
+
   ShellBrowserMainParts* shell_browser_main_parts_;
 
   DISALLOW_COPY_AND_ASSIGN(ShellContentBrowserClient);