[Policy] Make NewTabPageLocation take precedence over extensions

Previously, the NewTabPageLocation policy is lower-priority than
extensions' "chrome_url_overrides" setting. This is backwards, so
this patch fixes the issue.

TESTED=Installed Tabby Cat extension and set NewTabPageLocation policy.
       Manually opened a bunch of tabs before the patch (Tabby Cat) and
       after the patch (NewTabPageLocation)

Bug: 1020590
Change-Id: Ic3dbe9b9e0ed7efb318c85c489ba7f1faefbbb92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367976Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Reviewed-by: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Commit-Queue: Nicolas Ouellet-Payeur <nicolaso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800597}

chrome/browser/chrome_content_browser_client.cc

@@ -3558,6 +3558,12 @@ bool ChromeContentBrowserClient::OverrideWebPreferencesAfterNavigation(
 
 void ChromeContentBrowserClient::BrowserURLHandlerCreated(
     BrowserURLHandler* handler) {
+  // The group policy NTP URL handler must be registered before the other NTP
+  // URL handlers below. Also register it before the "parts" handlers, so the
+  // NTP policy takes precedence over extensions that override the NTP.
+  handler->AddHandlerPair(&HandleNewTabPageLocationOverride,
+                          BrowserURLHandler::null_handler());
+
   for (size_t i = 0; i < extra_parts_.size(); ++i)
     extra_parts_[i]->BrowserURLHandlerCreated(handler);
 
@@ -3568,11 +3574,6 @@ void ChromeContentBrowserClient::BrowserURLHandlerCreated(
   handler->AddHandlerPair(&WillHandleBrowserAboutURL,
                           BrowserURLHandler::null_handler());
 
-  // The group policy NTP URL handler must be registered before the other NTP
-  // URL handlers below.
-  handler->AddHandlerPair(&HandleNewTabPageLocationOverride,
-                          BrowserURLHandler::null_handler());
-
 #if defined(OS_ANDROID)
   // Handler to rewrite chrome://newtab on Android.
   handler->AddHandlerPair(&chrome::android::HandleAndroidNativePageURL,