Use ScopedTaskEnvironment instead of MessageLoopForUI in /ios/chrome

MessageLoopForUI will go away, eventually.

ScopedTaskEnvironment will per default start a ThreadPool, which should
be fine in most of the cases. If you believe your test needs to make sure
that no ThreadPool runs let me know and I will update the patch.

BUG=891670
This CL was uploaded by git cl split.

R=droger@chromium.org

Change-Id: Ieb7595218c6176d258dfbd7d83272a4b9d3704e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1717291
Auto-Submit: Carlos Caballero <carlscab@google.com>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Carlos Caballero <carlscab@google.com>
Cr-Commit-Position: refs/heads/master@{#683110}

ios/chrome/browser/installation_notifier_unittest.mm

@@ -3,13 +3,13 @@
 // found in the LICENSE file.
 
 #import "ios/chrome/browser/installation_notifier.h"
+#include "base/message_loop/message_loop_current.h"
 
 #include <stdint.h>
 #import <UIKit/UIKit.h>
 
 #include "base/ios/block_types.h"
-#include "base/message_loop/message_loop.h"
-#include "ios/web/public/test/test_web_thread.h"
+#include "ios/web/public/test/test_web_thread_bundle.h"
 #include "net/base/backoff_entry.h"
 #include "testing/platform_test.h"
 #import "third_party/ocmock/OCMock/OCMock.h"
@@ -92,9 +92,6 @@
 namespace {
 
 class InstallationNotifierTest : public PlatformTest {
- public:
-  InstallationNotifierTest() : ui_thread_(web::WebThread::UI, &message_loop_) {}
-
  protected:
   void SetUp() override {
     installationNotifier_ = [InstallationNotifier sharedInstance];
@@ -125,8 +122,7 @@ class InstallationNotifierTest : public PlatformTest {
                 50 + jitter * expectedDelayInMSec);
   }
 
-  base::MessageLoopForUI message_loop_;
-  web::TestWebThread ui_thread_;
+  web::TestWebThreadBundle thread_bundle_;
   __weak InstallationNotifier* installationNotifier_;
   __weak FakeDispatcher* dispatcher_;
   MockNotificationReceiver* notificationReceiver1_;