[iOS] Define BCVC presentation context if non-modal dialogs are enabled.

This will allow UIViewControllers presented on the BCVC to be presented
non-modally, leaving the browser controls in the toolbars unaffected.

Bug: 674649
Change-Id: Ib1c256965c4821074c544f5a87792203e2648208
Reviewed-on: https://chromium-review.googlesource.com/c/1334554
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608589}

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

@@ -27,6 +27,7 @@ source_set("ui") {
 
   deps = [
     "//base",
+    "//ios/chrome/browser/ui/dialogs:feature_flags",
     "//ios/chrome/browser/ui/fullscreen:ui",
     "//ios/chrome/browser/ui/util",
   ]

ios/chrome/browser/ui/browser_container/browser_container_view_controller.mm

@@ -5,6 +5,7 @@
 #import "ios/chrome/browser/ui/browser_container/browser_container_view_controller.h"
 
 #include "base/logging.h"
+#import "ios/chrome/browser/ui/dialogs/dialog_features.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
@@ -19,6 +20,8 @@
 - (void)viewDidLoad {
   [super viewDidLoad];
 
+  self.definesPresentationContext =
+      base::FeatureList::IsEnabled(dialogs::kNonModalDialogs);
   self.view.autoresizingMask =
       UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
 }