Set dispatcher in NTPToolbarController

The dispatcher wasn't set anymore due to some refactoring.
This CL sets it back.

Bug: 749488
Change-Id: I94a7d738721720dc3e2814f94d9658cd152debaa
Reviewed-on: https://chromium-review.googlesource.com/593567Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491286}

ios/chrome/browser/ui/ntp/new_tab_page_toolbar_controller.mm

@@ -57,9 +57,9 @@ enum {
 
 @implementation NewTabPageToolbarController
 
-@synthesize dispatcher = _dispatcher;
 @synthesize canGoForward = _canGoForward;
 @synthesize canGoBack = _canGoBack;
+@dynamic dispatcher;
 
 - (instancetype)initWithDispatcher:(id<ApplicationCommands,
                                        BrowserCommands,