[iOS] Implement TestFullscreenController::[Set/Get]WebStateList().

Bug: 932148
Change-Id: I24e6b181842913d8da3d02c10e1816636a01454d
Reviewed-on: https://chromium-review.googlesource.com/c/1490194
Auto-Submit: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635749}

ios/chrome/browser/ui/fullscreen/test/test_fullscreen_controller.h

@@ -45,6 +45,8 @@ class TestFullscreenController : public FullscreenController {
  private:
   // The model.
   FullscreenModel* model_ = nullptr;
+  // The WebStateList.
+  WebStateList* web_state_list_ = nullptr;
   // The broadcaster.
   ChromeBroadcaster* broadcaster_ = nil;
   // The observers.

ios/chrome/browser/ui/fullscreen/test/test_fullscreen_controller.mm

@@ -23,14 +23,16 @@ ChromeBroadcaster* TestFullscreenController::broadcaster() {
   return broadcaster_;
 }
 
-void TestFullscreenController::SetWebStateList(WebStateList* web_state_list) {}
+void TestFullscreenController::SetWebStateList(WebStateList* web_state_list) {
+  web_state_list_ = web_state_list;
+}
 
 const WebStateList* TestFullscreenController::GetWebStateList() const {
-  return nullptr;
+  return web_state_list_;
 }
 
 WebStateList* TestFullscreenController::GetWebStateList() {
-  return nullptr;
+  return web_state_list_;
 }
 
 void TestFullscreenController::AddObserver(