[ios] Presents bookmarks context menus without using the root VC.

Bookmarks was using the window's rootViewController to present context menus.
This strategy does not work when BVC is presented instead of contained, so
bookmarks is updated to present context menus on top of |self| instead.  For
safety, this change only takes effect when the BVC presentation experiment is
enabled.

BUG=768563

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ib129197b1aece67b2cc55d58de8e9be79d0af06e
Reviewed-on: https://chromium-review.googlesource.com/760481
Commit-Queue: Rohit Rao (ping after 24h) <rohitrao@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515773}

ios/chrome/browser/ui/bookmarks/BUILD.gn

@@ -128,6 +128,7 @@ source_set("bookmarks") {
     "//ios/chrome/browser/ui/commands",
     "//ios/chrome/browser/ui/icons",
     "//ios/chrome/browser/ui/keyboard",
+    "//ios/chrome/browser/ui/main:feature_flags",
     "//ios/chrome/browser/ui/material_components",
     "//ios/chrome/browser/ui/ntp",
     "//ios/chrome/browser/ui/signin_interaction/public",

ios/chrome/browser/ui/bookmarks/bookmark_home_tablet_ntp_controller.mm

@@ -34,6 +34,7 @@
 #import "ios/chrome/browser/ui/bookmarks/bookmark_panel_view.h"
 #import "ios/chrome/browser/ui/bookmarks/bookmark_promo_controller.h"
 #import "ios/chrome/browser/ui/bookmarks/bookmark_utils_ios.h"
+#include "ios/chrome/browser/ui/main/main_feature_flags.h"
 #import "ios/chrome/browser/ui/rtl_geometry.h"
 #import "ios/chrome/browser/ui/uikit_ui_util.h"
 #include "ios/chrome/grit/ios_strings.h"
@@ -272,8 +273,12 @@ const CGFloat kNavigationBarTopMargin = 8.0;
 }
 
 - (ActionSheetCoordinator*)createActionSheetCoordinatorOnView:(UIView*)view {
+  UIViewController* baseViewController =
+      TabSwitcherPresentsBVCEnabled() ? self
+                                      : self.view.window.rootViewController;
+
   return [[ActionSheetCoordinator alloc]
-      initWithBaseViewController:self.view.window.rootViewController
+      initWithBaseViewController:baseViewController
                            title:nil
                          message:nil
                             rect:view.bounds