[Mac] Fix regression where commands were disabled in full screen.

This was caused by: https://codereview.chromium.org/1250403002

That CL moved where command validation happened, but the new
handler wasn't hooked up to the full screen window that gets
opened on 10.6.

Note: The 10.6 behavior can be simulated by changing the
return value of SupportsSystemFullscreen() to false.

BUG=569418
TEST=On Mac OS 10.6, check that cmd-shift-F can toggle full
screen back and forth.

Review URL: https://codereview.chromium.org/1520283003

Cr-Commit-Position: refs/heads/master@{#365375}

chrome/browser/ui/cocoa/browser_window_controller.mm

@@ -184,6 +184,17 @@
 using content::RenderWidgetHostView;
 using content::WebContents;
 
+namespace {
+
+void SetUpBrowserWindowCommandHandler(NSWindow* window) {
+  // Make the window handle browser window commands.
+  [base::mac::ObjCCastStrict<ChromeEventProcessingWindow>(window)
+      setCommandHandler:[[[BrowserWindowCommandHandler alloc] init]
+                            autorelease]];
+}
+
+}  // namespace
+
 @interface NSWindow (NSPrivateApis)
 // Note: These functions are private, use -[NSObject respondsToSelector:]
 // before calling them.
@@ -227,10 +238,7 @@ - (id)initWithBrowser:(Browser*)browser takeOwnership:(BOOL)ownIt {
     browser_.reset(browser);
     ownsBrowser_ = ownIt;
     NSWindow* window = [self window];
-    // Make the window handle browser window commands.
-    [base::mac::ObjCCastStrict<ChromeEventProcessingWindow>(window)
-        setCommandHandler:[[[BrowserWindowCommandHandler alloc] init]
-                              autorelease]];
+    SetUpBrowserWindowCommandHandler(window);
 
     // Make the content view for the window have a layer. This will make all
     // sub-views have layers. This is necessary to ensure correct layer
@@ -1448,8 +1456,10 @@ - (void)addFindBar:(FindBarCocoaController*)findBarCocoaController {
 }
 
 - (NSWindow*)createFullscreenWindow {
-  return [[[FullscreenWindow alloc] initForScreen:[[self window] screen]]
-           autorelease];
+  NSWindow* window = [[[FullscreenWindow alloc]
+      initForScreen:[[self window] screen]] autorelease];
+  SetUpBrowserWindowCommandHandler(window);
+  return window;
 }
 
 - (NSInteger)numberOfTabs {