Similar as extension, DOM UI needs to be created in its own process....

chrome/browser/renderer_host/test/render_process_host_browsertest.cc

 // Copyright (c) 2010 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 "base/command_line.h"
 #include "chrome/browser/renderer_host/site_instance.h"
 #include "chrome/browser/tab_contents/tab_contents.h"
 #include "chrome/browser/renderer_host/render_process_host.h"
 #include "chrome/browser/ui/browser.h"
+#include "chrome/common/chrome_switches.h"
 #include "chrome/common/url_constants.h"
 #include "chrome/test/in_process_browser_test.h"
 #include "chrome/test/ui_test_utils.h"
 
 class RenderProcessHostTest : public InProcessBrowserTest {
  public:
   RenderProcessHostTest() {
     EnableDOMAutomation();
   }
 
   int RenderProcessHostCount() {
     RenderProcessHost::iterator hosts = RenderProcessHost::AllHostsIterator();
     int count = 0;
     while (!hosts.IsAtEnd()) {
       if (hosts.GetCurrentValue()->HasConnection())
         count++;
       hosts.Advance();
     }
     return count;
   }
 };
 
+IN_PROC_BROWSER_TEST_F(RenderProcessHostTest, ProcessPerTab) {
+  // Set max renderers to 1 to force running out of processes.
+  RenderProcessHost::SetMaxRendererProcessCount(1);

[Charlie Reis, 2011/01/06 18:27:59]

I don't think you need this line.  Even if the max renderer process count is 20,
your change should make it so that opening two New Tab Pages share the same
process.

[klobag.chromium, 2011/01/06 20:23:02]

I added some normal page cases.

+
+  CommandLine& parsed_command_line = *CommandLine::ForCurrentProcess();
+  parsed_command_line.AppendSwitch(switches::kProcessPerTab);
+
+  int tab_count = 1;
+
+  // Change the first tab to be the new tab page (TYPE_DOMUI).
+  GURL newtab(chrome::kChromeUINewTabURL);
+  ui_test_utils::NavigateToURL(browser(), newtab);
+  EXPECT_EQ(tab_count, browser()->tab_count());
+  EXPECT_EQ(1, RenderProcessHostCount());
+
+  // Create another new tab.  It should share the previous process.
+  browser()->NewTab();
+  if (browser()->tab_count() == tab_count)
+    ui_test_utils::WaitForNewTab(browser());
+  tab_count++;
+  EXPECT_EQ(tab_count, browser()->tab_count());
+  EXPECT_EQ(1, RenderProcessHostCount());
+}
+
 // When we hit the max number of renderers, verify that the way we do process
 // sharing behaves correctly.  In particular, this test is verifying that even
 // when we hit the max process limit, that renderers of each type will wind up
 // in a process of that type, even if that means creating a new process.
 // TODO(erikkay) crbug.com/43448 - disabled for now until we can get a
 // reasonable implementation in place.
 IN_PROC_BROWSER_TEST_F(RenderProcessHostTest, DISABLED_ProcessOverflow) {
   // Set max renderers to 1 to force running out of processes.
   RenderProcessHost::SetMaxRendererProcessCount(1);
 
@@ skipping 64 matching lines @@
   tab_count++;
   host_count++;
   EXPECT_EQ(tab_count, browser()->tab_count());
   tab1 = browser()->GetTabContentsAt(tab_count - 1);
   rph3 = tab1->GetRenderProcessHost();
   EXPECT_EQ(tab1->GetURL(), bookmarks);
   EXPECT_EQ(host_count, RenderProcessHostCount());
   EXPECT_NE(rph1, rph3);
   EXPECT_NE(rph2, rph3);
 }