Redirect chrome://welcome to the new tab page.

The welcome page was removed in M40. Users who had the welcome page open
when upgrading from pre-M40 to post-M40 will see a "webpage not
available" error after the upgrade. This fixes that behavior by
replacing the welcome page with an NTP.

BUG=455427

Review URL: https://codereview.chromium.org/1036533002

Cr-Commit-Position: refs/heads/master@{#322049}

chrome/browser/android/new_tab_page_url_handler.cc

@@ -13,6 +13,7 @@
 
 namespace {
 const char kBookmarkFolderPath[] = "folder/";
+const char kLegacyWelcomeHost[] = "welcome";
 }
 
 namespace chrome {
@@ -20,10 +21,14 @@ namespace android {
 
 bool HandleAndroidNativePageURL(GURL* url,
                                 content::BrowserContext* browser_context) {
-  if (url->SchemeIs(content::kChromeUIScheme) &&
-      url->host() == chrome::kChromeUINewTabHost) {
-    *url = GURL(chrome::kChromeUINativeNewTabURL);
-    return true;
+  if (url->SchemeIs(content::kChromeUIScheme)) {
+    // TODO(newt): stop redirecting chrome://welcome to chrome-native://newtab
+    // when M39 is a distant memory. http://crbug.com/455427
+    if (url->host() == chrome::kChromeUINewTabHost ||
+        url->host() == kLegacyWelcomeHost) {
+      *url = GURL(chrome::kChromeUINativeNewTabURL);
+      return true;
+    }
   }
 
   if (url->SchemeIs(chrome::kChromeNativeScheme) &&

chrome/common/url_constants.h

@@ -255,10 +255,6 @@ extern const char kChromeUIThemePath[];
 extern const char kChromeUIPrintHost[];
 #endif  // ENABLE_PRINT_PREVIEW
 
-#if defined(OS_ANDROID)
-extern const char kChromeUIWelcomeHost[];
-#endif
-
 #if defined(OS_POSIX) && !defined(OS_MACOSX) && !defined(OS_ANDROID)
 extern const char kChromeUILinuxProxyConfigHost[];
 extern const char kChromeUISandboxHost[];