Create NTP tab when split view is created with active tab

fix brave/brave-browser#46191

Uptream loads split view specific page when createing another tab.
We want to load NTP instead.

TEST=SplitViewLocationBarBrowserTest.URLShouldBeUpdated_WhenActiveTabChanges

1. launch browser with --enable-features=SideBySide
2. Select "Add tab to split view" from active tab's context menu
3. Check NTP is loaded in newly created tab

Author: simonhong

browser/ui/views/split_view/split_view_location_bar_browsertest.cc

@@ -134,14 +134,6 @@ IN_PROC_BROWSER_TEST_P(SplitViewLocationBarBrowserTest,
             tab_strip_model().GetWebContentsAt(0)->GetVisibleURL().spec());
   ASSERT_EQ(u"about:blank", split_view_location_bar().url_->GetText());
 
-  // For now upstream's another page is loaded when split view is created
-  // with active tab. So, loaded newtab to use below url check.
-  if (IsSideBySideEnabled()) {
-    ASSERT_TRUE(ui_test_utils::NavigateToURLWithDisposition(
-        browser(), GURL("chrome://newtab/"), WindowOpenDisposition::CURRENT_TAB,
-        ui_test_utils::BROWSER_TEST_WAIT_FOR_LOAD_STOP));
-  }
-
   // When activating another tab in split view,
   tab_strip_model().ActivateTabAt(0);
 

chromium_src/chrome/browser/ui/browser_commands.cc

@@ -8,10 +8,13 @@
 #include "brave/components/commander/common/buildflags/buildflags.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/ui/browser_commands.h"
+#include "chrome/common/webui_url_constants.h"
 
 #define ReloadBypassingCache ReloadBypassingCache_ChromiumImpl
 #define GetReadingListModel GetReadingListModel_ChromiumImpl
+#define kChromeUISplitViewNewTabPageURL kChromeUINewTabURL
 #include "src/chrome/browser/ui/browser_commands.cc"
+#undef kChromeUISplitViewNewTabPageURL
 #undef ReloadBypassingCache
 #undef GetReadingListModel