[PM] Remove expectation that DidStartLoading/DidStopLoading are invoked in...

[PM] Remove expectation that DidStartLoading/DidStopLoading are invoked in alternance in PageLoadMetricsWebContentsObserver.

R=chrisha@chromium.org

Bug: 1145482, 1145572
Change-Id: Icedfcd1675de8751cf17ee44d396090270131ced
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519478
Auto-Submit: François Doray <fdoray@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824036}

chrome/browser/performance_manager/observers/page_load_metrics_observer.cc

@@ -179,8 +179,12 @@ bool PageLoadMetricsWebContentsObserver::IsDevTools() const {
 }
 
 void PageLoadMetricsWebContentsObserver::DidStartLoading() {
-  DCHECK(!is_loading_);
   DCHECK(web_contents()->IsLoading());
+
+  // TODO(crbug.com/1145572): Uncomment this DCHECK once there is a guarantee
+  // that DidStartLoading and DidStopLoading are invoked in alternance.
+  // DCHECK(!is_loading_);
+
   is_loading_ = true;
 }