[Mac] User manager should open on the screen that has focus

(vs always opening on the first screen like it's happening now)

BUG=405602
TEST=Start Chrome with --enable-new-avatar-menu (if it's not own by default). If you've got to screens, when you press "Switch users" the User Manager should show up on whatever screen you triggered it from.

Review URL: https://codereview.chromium.org/489953002

Cr-Commit-Position: refs/heads/master@{#291201}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291201 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/ui/cocoa/profiles/user_manager_mac.mm

@@ -92,11 +92,10 @@ class UserManagerWebContentsDelegate : public content::WebContentsDelegate {
 - (id)initWithProfile:(Profile*)profile
          withObserver:(UserManagerMac*)userManagerObserver {
 
-  // Center the window on the primary screen.
-  CGFloat screenHeight =
-      [[[NSScreen screens] objectAtIndex:0] frame].size.height;
-  CGFloat screenWidth =
-      [[[NSScreen screens] objectAtIndex:0] frame].size.width;
+  // Center the window on the screen that currently has focus.
+  NSScreen* mainScreen = [NSScreen mainScreen];
+  CGFloat screenHeight = [mainScreen frame].size.height;
+  CGFloat screenWidth = [mainScreen frame].size.width;
 
   NSRect contentRect = NSMakeRect((screenWidth - kWindowWidth) / 2,
                                   (screenHeight - kWindowHeight) / 2,
@@ -107,7 +106,8 @@ class UserManagerWebContentsDelegate : public content::WebContentsDelegate {
                           NSClosableWindowMask |
                           NSResizableWindowMask
                   backing:NSBackingStoreBuffered
-                    defer:NO];
+                    defer:NO
+                   screen:mainScreen];
   [window setTitle:l10n_util::GetNSString(IDS_PRODUCT_NAME)];
   [window setMinSize:NSMakeSize(kWindowWidth, kWindowHeight)];