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

chrome/browser/renderer_host/test/render_process_host_browsertest.cc

Index: chrome/browser/renderer_host/test/render_process_host_browsertest.cc
===================================================================
--- chrome/browser/renderer_host/test/render_process_host_browsertest.cc	(revision 70598)
+++ chrome/browser/renderer_host/test/render_process_host_browsertest.cc	(working copy)
@@ -2,10 +2,12 @@
 // 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"
@@ -28,6 +30,30 @@
   }
 };
 
+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