[ios] Fix DCHECK in reportBackUserAction

Do not call reportBackUserAction on SettingsTableViewController as root
settings view controller does not have "back" button and does not have
to record "back" metric.

DCHECK was hapenning after dragging SettingsTableViewController bottom
and returning it back, which called presentation callback for root
settings controller.

Bug: 1117512
Change-Id: I45f3a2285007e8098b9155501a9c526fecf1df8f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363003
Auto-Submit: Eugene But <eugenebut@chromium.org>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799734}

ios/chrome/browser/ui/settings/settings_navigation_controller.mm

@@ -465,9 +465,11 @@ NSString* const kSettingsDoneButtonId = @"kSettingsDoneButtonId";
       willShowViewController:(UIViewController*)viewController
                     animated:(BOOL)animated {
   if ([viewController isMemberOfClass:[SettingsTableViewController class]] &&
+      ![self.currentPresentedViewController
+          isMemberOfClass:[SettingsTableViewController class]] &&
       [self.currentPresentedViewController
           conformsToProtocol:@protocol(SettingsControllerProtocol)]) {
-    // Navigated back to SettingsTableViewController.
+    // Navigated back to root SettingsController from leaf SettingsController.
     [self.currentPresentedViewController
         performSelector:@selector(reportBackUserAction)];
   }