[iOS][DarkMode] Support dark mode in collection view controller

MDCCollectionView uses a white background by default. This overrides
the default color with one that supports dark mode.

Bug: 976675
Change-Id: I3b2f6688698c90f29093cb17dbc9d30e9e3a45ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1741913Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Auto-Submit: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685513}

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

@@ -14,6 +14,7 @@ source_set("collection_view") {
     "//base",
     "//ios/chrome/browser/ui/list_model",
     "//ios/chrome/browser/ui/material_components",
+    "//ios/chrome/common/colors",
   ]
   public_deps = [
     "//ios/chrome/browser/ui/collection_view/cells",

ios/chrome/browser/ui/collection_view/collection_view_controller.mm

@@ -11,6 +11,7 @@
 #import "ios/chrome/browser/ui/collection_view/collection_view_model.h"
 #import "ios/chrome/browser/ui/material_components/chrome_app_bar_view_controller.h"
 #import "ios/chrome/browser/ui/material_components/utils.h"
+#import "ios/chrome/common/colors/UIColor+cr_semantic_colors.h"
 #import "ios/third_party/material_components_ios/src/components/CollectionCells/src/MaterialCollectionCells.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
@@ -61,6 +62,9 @@
                name:UIContentSizeCategoryDidChangeNotification
              object:nil];
   }
+
+  // Suport dark mode.
+  self.collectionView.backgroundColor = UIColor.cr_systemGroupedBackgroundColor;
 }
 
 - (void)contentSizeCategoryDidChange:(id)sender {