Fix TestBrowserThreadBundle initialization in PlatformNotificationContextTriggerTest

I initialized the TestBrowserThreadBundle with REAL_IO_THREAD in
crrev.com/c/1658644 to get around some fuchsia-specific behavior. That
wasn't the correct way to fix it, this CL is.

Bug: 974365
Change-Id: I55a2d533654f15d9280215bf26e652e1163be74b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1662968Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Commit-Queue: Robbie McElrath <rmcelrath@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670147}

content/browser/notifications/platform_notification_context_trigger_unittest.cc

@@ -53,12 +53,9 @@ class PlatformNotificationContextTriggerTest : public ::testing::Test {
  public:
   PlatformNotificationContextTriggerTest()
       : thread_bundle_(
-            base::test::ScopedTaskEnvironment::MainThreadType::MOCK_TIME,
-            base::test::ScopedTaskEnvironment::NowSource::MAIN_THREAD_MOCK_TIME,
-            // TODO(crbug.com/974365): The IO thread is only needed so
-            // MessagePumpFuchsia will create an async::Loop, which is needed
-            // for fidl.
-            content::TestBrowserThreadBundle::REAL_IO_THREAD),
+            base::test::ScopedTaskEnvironment::MainThreadType::UI_MOCK_TIME,
+            base::test::ScopedTaskEnvironment::NowSource::
+                MAIN_THREAD_MOCK_TIME),
         notification_browser_client_(&browser_context_),
         success_(false) {
     SetBrowserClientForTesting(&notification_browser_client_);