[ios] Remove DCHECK from dispatchInstallationNotifierBlock

DownloadManagerCoordinatorTest.CloseInProgressDownload test fails,
because dispatchInstallationNotifierBlock schedules a delayed block
execution and that block expect proper setup of WebThread. The
expectation flakily fails if tests tear down WebThread environment.

The expectation to have WebThread set up is incorrect for delayed block
of code in tests and production, because WebThread can be legitimately
torn down.

This CL removes that DCHECK, but keeps all other DCHECKs that are still
active as long as InstallationNotifier object is alive.

Bug: 1059885
Change-Id: I9b4b257a5b654923d52840bb314d36ff243269e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2095349
Auto-Submit: Eugene But <eugenebut@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748703}

ios/chrome/browser/installation_notifier.mm

@@ -161,7 +161,6 @@ const net::BackoffEntry::Policy kPollingBackoffPolicy = {
   __weak InstallationNotifier* weakSelf = self;
   [_dispatcher dispatchAfter:delayInNSec
                    withBlock:^{
-                     DCHECK_CURRENTLY_ON(web::WebThread::UI);
                      InstallationNotifier* strongSelf = weakSelf;
                      if (blockId == [strongSelf lastCreatedBlockId]) {
                        [strongSelf pollForTheInstallationOfApps];