Removed errorneous DCHECK from TaskManager SharedSampler code

The DCHECK could be triggered if sampling of results by SharedSampler on
background thread took longer than refresh cycle (1 second) and at the
same time user added or removed one of TaskManager columns handled by
SharedSampler.

This DCHECK was an errorneous assumption. I removed it and added a comment
explaining this situation.

BUG=678471

Review-Url: https://codereview.chromium.org/2612773004
Cr-Commit-Position: refs/heads/master@{#441958}

chrome/browser/task_manager/sampling/shared_sampler_win.cc

@@ -369,9 +369,15 @@ void SharedSampler::Refresh(base::ProcessId process_id, int64_t refresh_flags) {
         base::Bind(&SharedSampler::RefreshOnWorkerThread, this),
         base::Bind(&SharedSampler::OnRefreshDone, this));
   } else {
+    // http://crbug.com/678471
     // A group of consecutive Refresh calls should all specify the same refresh
-    // flags.
-    DCHECK_EQ(refresh_flags, refresh_flags_);
+    // flags. Rarely RefreshOnWorkerThread could take a long time (> 1 sec),
+    // long enough for a next refresh cycle to start before results are ready
+    // from a previous cycle. In that case refresh_flags_ would still remain
+    // set to the previous cycle refresh flags which might be different than
+    // this cycle refresh flags if a column was added or removed between the two
+    // cycles. The worst that could happen in that condition is that results for
+    // a newly added column would be missing for one extra refresh cycle.
   }
 
   refresh_flags_ |= refresh_flags;