Remove the wrong DCHECK in MetricsWebContentsObserver

MetricsWebViewContentObserver assumed WillStartNavigationRequest()
and DidFinishNavigation() is sequencially called for main frame, it
isn't true, since both callbacks are post task, calling sequence
isn't guaranteed.

Bug: 1010034
Bug: 937528
Change-Id: Ia7fc892a599f789aa56b613409815f45d36e4223
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1865268Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Commit-Queue: Tao Bai <michaelbai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709121}

components/page_load_metrics/browser/metrics_web_contents_observer.cc

@@ -221,10 +221,6 @@ void MetricsWebContentsObserver::WillStartNavigationRequestImpl(
   const GURL& currently_committed_url =
       committed_load_ ? committed_load_->url() : opener_url;
 
-  // We can have two provisional loads in some cases. E.g. a same-site
-  // navigation can have a concurrent cross-process navigation started
-  // from the omnibox.
-  DCHECK_GT(2ul, provisional_loads_.size());
   // Passing raw pointers to observers_ and embedder_interface_ is safe because
   // the MetricsWebContentsObserver owns them both list and they are torn down
   // after the PageLoadTracker. The PageLoadTracker does not hold on to