RC: Tiny cleanup in TabLoadTracker.

Bug: 854598
Change-Id: I457e8940a4ad5ff4aa89cfae4415f0addd1e6a64
Reviewed-on: https://chromium-review.googlesource.com/1118621
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571171}

chrome/browser/resource_coordinator/tab_load_tracker.cc

@@ -164,11 +164,11 @@ void TabLoadTracker::DidFailLoad(content::WebContents* web_contents) {
 void TabLoadTracker::OnPageAlmostIdle(content::WebContents* web_contents) {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
   DCHECK(resource_coordinator::IsPageAlmostIdleSignalEnabled());
-  // PageAlmostIdle signals can be arbitrarily delayed as they are asynchronous.
-  // As such, they can arrive after the web contents in question no longer
-  // exists.
-  if (!base::ContainsKey(tabs_, web_contents))
-    return;
+  // TabManager::ResourceCoordinatorSignalObserver filters late notifications
+  // so here we can assume the event pertains to a live web_contents and
+  // its most recent navigation.
+  DCHECK(base::ContainsKey(tabs_, web_contents));
+
   MaybeTransitionToLoaded(web_contents);
 }