[Mac] Allow using the arrow keys for tabbing in the avatar bubble.

BUG=414794

Review URL: https://codereview.chromium.org/589263002

Cr-Commit-Position: refs/heads/master@{#296288}

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

@@ -1042,6 +1042,14 @@ class ActiveProfileObserverBridge : public AvatarMenuObserver,
   [super windowWillClose:notification];
 }
 
+- (void)moveDown:(id)sender {
+  [[self window] selectNextKeyView:self];
+}
+
+- (void)moveUp:(id)sender {
+  [[self window] selectPreviousKeyView:self];
+}
+
 - (void)cleanUpEmbeddedViewContents {
   webContents_.reset();
 }