--install-chrome-app opens new tab in existing window.

chrome/browser/apps/install_chrome_app.cc

 // 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.
 
 #include "chrome/browser/apps/install_chrome_app.h"
 
 #include "base/basictypes.h"
 #include "base/command_line.h"
 #include "base/strings/stringprintf.h"
 #include "chrome/browser/browser_process.h"
@@ skipping 23 matching lines @@
 }
 
 void NavigateToUrlWithAccountsReferrer(const GURL& url) {
   Browser* browser =
       BrowserList::GetInstance(chrome::HOST_DESKTOP_TYPE_NATIVE)->get(0);
   if (!browser)
     return;
 
   chrome::NavigateParams params(
       browser, url, content::PAGE_TRANSITION_AUTO_TOPLEVEL);
+  params.disposition = NEW_FOREGROUND_TAB;
   params.window_action = chrome::NavigateParams::SHOW_WINDOW;
   params.referrer = content::Referrer();
   params.referrer.url = GURL(kAccountsUrl);
   chrome::Navigate(&params);
 }
 
 class AppURLFetcher : net::URLFetcherDelegate {
  public:
   explicit AppURLFetcher(const std::string& app_id);
 
@@ skipping 33 matching lines @@
 namespace install_chrome_app {
 
 void InstallChromeApp(const std::string& app_id) {
   if (!extensions::Extension::IdIsValid(app_id))
     return;
 
   new AppURLFetcher(app_id);
 }
 
 }  // namespace install_chrome_app